I am a trombone player, I'm young (15) so I don't have much money, I've been playing on a student sized horn (yamaha YSL354) but many trombone players and my band director say that a student sized horn will only hold me back, they think I should be playing on a full professional sized horn, so does anybody know where I could find a professional sized horn (i,e. conn 8h, or bach 42b, etc.) in fairly good shape for somewhere around/under 500 dollars?

You won't find a professional horn for that price. Bach 42b's and Conn 88H's usually sell used for around 1500$. Very good ones will go for 2000$ (and sometimes more). Around 1000$ is a bargain. Under that, I would suspect some scam or fraud.

Keep in mind that although the small bore may hold you back (this is very relative, as there are pro grade small bore horns - mostly for jazz) as far as tone darkness and power, the switch from small bore to symphonic bore (and especially to a Bach 42) is a big step. How long have you been playing for?

You MAY want to play on a medium bore horn for a year or two before switching to a large bore. A YSL-356 would feel pretty much like your 354, while allowing you to make a smoother transition to large bore. Also, it would allow you to learn to use the F-attachment. There are other suitable intermediate medium bore horns, but I like to stick with Yamaha as far as student and intermediate instruments go. They are very reliable and sturdy instruments, and you can't go wrong with them.

You could be able to find a used 356 for between 500 and 700$ in an instruments store or on eBay, while you save up more money in order to buy your professional horn. Also keep in mind that used instruments don't lose their value when you buy them, so you could easily sell a used horn for as much (or even more) than what you paid for it.