How do I choose a replacement Derailleur? I have a Psycle Werks downhill-bike and the derailleur got bent and cracked I need it to get replaced does anyone know or have any suggestions on where I can get a new one or what I should do?
Let's say that your new derailleur is dependent on the number of gears you have on the back and sometimes brand dependent. SRAM and Shimano, have (they used to have at least) different pull ratios and/or inverted numbers (which is not a problem, only a funny fact), so this are the first thing to check out.

Then as you said you do DH, so you need in my opinion a short, or medium length derailleur; so it doesn't bounce that much and also to have a little more clearance for obstacles.

That said there are some derailleurs that have clutch systems so the cage won't move as you hit bumps and obstacles, for a more consistent pedaling.

If you have the money go for a Shimano Saint, Hone or SLX. Then again if you don't have it, go for a Shimano XT 98. If you have a small ratio transmission in the back (tooth count doesn't differ that much from gear to gear) you can get a away with a derailleur made for road bike, small and light.

PS: You can get your old derailleur straight with the use of two french-wrenches (crescent wrench), one to set the direction to bend and the other to apply force to the first one.

Cheers.