300 is probably the bare minimum that you want to spend... There's a few under that I would recommend, but technically you have to prove that you are 18 or older by becoming age veriified like the guys have mentioned. If you have strong google fu, you can find the guns I would recommend.
Don't try ordering from the states, unless you know the exact procedure, customs tends to seize them and you'll be out 300 bucks.
btw, at 300$ you'll have your work cut out for you.. that's right about the line that crap starts turning into decent, so you will need to do some good research to find out which are the good ones.
I believe we can tell you what brands are good, but we can't tell you where to get them.