there are AUGs for sale in the classifieds.
You can't access classifieds because you need to meet an age-verifier before.
sign up for a game in the events section of this site and you'll be able to meet one.
then you can buy used and new guns, but they are more than 200$.
any AEG is more than 350$ in Canada.
if you are less than 18 you can't play, you can't buy.