First thing, lets clarify something, a sniper is someone who hides at a distance and shoots with a precision rifle and is never on the front lines. A common misconception is that a sniper will be the one to get the most amount of kills, this is rarely the case. It is far more common for a sniper to get few kills but typically rather important ones. They also often act as spotters for the rest of the team, reporting on movement of the opposing teams because of the range of their scopes. There are many different loadouts for snipers.
Maruzen Type 96
There are three most popular sniper rifles, the first and most popular is the VSR platform. The upgrades and parts are available to find almost anywhere. The second most used platform is the APS2 Maruzen type 96 rifle. This varies in build from the L96 platform, despite the similar name. The type 96 platform is a platform that can be very hard to find upgrades for, the Action Army upgrades are the most readily available upgrades for this particular platform. The third most common platform is the Tokyo Marui L96 rifle. These have a more realistic look to them with the enlarged mag closer to the trigger of the rifle.
Generally, stock snipers are built with many plastic parts. The higher end stock guns (like Tokyo Marui and APS Maruzen) will last longer than their clones (like UTG and Well) but they will all eventually break.
It is good to replace the plastic parts with upgrade parts as fast as possible. The most important upgrades in order are as follows: trigger assembly, piston, spring guide, hop up unit, bucking and nub (nubbing), barrel, cylinder, spring. This can vary based on the platform but this is generally what breaks first. The first three (trigger, piston, spring guide) are almost always plastic and should be replaced quickly, the rest of the parts are generally metal (although quality isn't the best). A common misconception is that you should replace the spring first, that is generally not a good idea. If you are planning on building you gun to shoot 550 fps with .2s and you put in a spring to get you there and then change the barrel and hop up unit (which seals the gun much better) you gun might now shoot at 600 fps. Generally, you should replace the spring last to get to your desired fps.
These are the brand of upgrades that are widely considered the best brands in order: Laylax, PDI, EDGI, Action Army. Laylax is particularly known for their pistons, springs, and spring guides. PDI is particularly known for their cylinders. EDGI is known for making the best barrels on the market. Action Army is known for making great trigger assemblies and hop up units. Even though this is what they are most famous for, they do make other great parts.
There are many different apparel methods for snipers which depend largely on the environment they play in. Stereotypical snipers wear ghillie suits, the problem with ghillie suits, and the reason they are not that common, is how thick they are. This causes problems with use in warmer climates as well as movement. The thickness makes it much like a winter coat, if you wear one on a hot summer day, you are at risk of overheating and collapsing. If you do collapse in a ghillie suit it might be a long time before anyone finds you, these should be used with caution. However, ghillie suits are generally considered the best form of camouflage. The threads on a ghillie suit causes it to blend with the environment better than most other forms of camo.
Another apparel method which has steadily become more popular is the 3D suits. These are mesh suits with cloth leaves sown onto them. These are particularly helpful because they can camouflage better than most camo techniques (excluding ghillie suits) yet they are lighter and cooler than ghillie suites. Most other camouflage techniques are just variations on camo patterns such as multicam with no 3D help at all. Sometimes, non 3D can be very effective as well. The most common non 3D camos are multicam, which was created for the military and designed to blend into nearly every environment, and hunting camo, which is similar to multicam.
C02 Pistol Taurus M9
Snipers prefer many different types of sidearms. Many will take a C02 or propane pistol onto the field with them, some will build their own stubby M4 or Mp5. C02 and propane being the most popular versions of pistols anyways inherently makes them some of the most popular sniper sidearms. Some snipers have switched to short, or stubby, M4s because in a bad situation, an M4 will be able to put out more BBs faster and more accurately than just about any pistol. Other short guns can serve the same purpose although M4s are the most popular small machine gun used as sidearms because the parts are easy to find and the mags are shorter yet hold more ammo then some other machine gun mags such as the Mp5.
Some of the less known or used sidearms than can be just as effective or even more so are the spring pistols and AEPs. Spring pistols are often laughed at or not taken seriously, however, a high quality spring pistol can get really good kills. The advantages of a spring pistol over other forms or pistols are: good in any temperature, silent, do not run on any propellant, and can generally hold more ammo than gas pistols. These are often useful particularly for snipers because the silence of them can allow for a close quarters kill without giving away your position. AEPs or Airsoft electric pistols have a similar effect. They do run on an external power source but a single battery for these guns can last a long time and are very small so they can be easily replaced in the field. The mags are far smaller than most gas pistol mags and therefore, many more can be carried in the same space. They are also quieter than gas or propane guns. They do have cons though, they typically don't have very much power, range or accuracy. They are strictly close quarters guns.