^Grizzly is right. We won't be able to give you any information on where to buy guns until you are Age Verified (proved that you are over 18 years of age). Please have a read through the FAQs to find your nearest AV representative, meet up and provide some ID. That way you'll get access to a whole lot of retailers who can pretty much get you any gun you want. You'll also get access to the classifieds where you can find other members selling guns and gear.
If you plan on attending public games and such, a lot of people would also recommend that you do NOT get a sniper rifle as your first gun. While a decent sniper rifle costs about $400-500 dollars, you'll have to put in a significant amount of upgrades before you'll have any advantage in range/accuracy over other people using AEGs (automatic electric gun) and gas guns. Anywhere from $400-1000 in additional modifications and such. Otherwise you'll have the same or less range than others using electric/gas guns and they'll have the advantage of full-auto fire, which will leave you severely outgunned. A better option would be to get a DMR type AEG which you can use in semi-auto to snipe, and switch to full auto if you find yourself needing more firepower.
If you would rather stick with a spring sniper rifle, get AV'ed, and you'll be able to get your hands on whatever you wish.
H&K G36c (KWA) / Glock 18c (TM) / Kimber LAPD SWAT Custom (TM) / Mossberg M500 (ACM)