I honestly fell like such a lanky person on my bmx, I went(for the first time!) to one of my nearby DH bike parks and noticed they had a slopestyle/ dirt jumping track that looked amazing. So now I'm looking for a DJ bike, but I really dont have too much to spend, maybe around $600? I realize its not that much.. I was looking at the GT bump "26
If you want to buy a new bike, if you get a complete that comes with a rigid fork instead of a suspension fork, you'll save some money, because a typical good DJ suspension fork is $300-450 for just the fork, while a rigid fork is a similar price as BMX forks.
pinkbike and craigslist are going to be your best bet. look out for a lightly used bike. you don't want to have to replace the fork, because that kinda kills the value.