Good idea for a bong.

as long as you buy it as a water pipe, its legal. Don't call it a bong when your buying it, or else they can't sell it to you.
You can order one offline I'm sure without any problem, a lot more shadey stuff goes through the mail all the time.
I'd just go find a head shop. sometimes they are also called "Smoke Shops"
if for whatever reason you can't find one, find a cigarette store, they should have them.
And if not, you can make a bong very easily.

 

ima say bad idea for a bong.. just grab a water bottle, and make a bong out of it.. or better yet.. make a bucket! and for a bowl use a socket if you got one

 

what is a socket?