I'm looking for an image of a gorilla beating its chest and roaring.
Most microstock photographers don't get paid enough to contemplate taking that kind of shot. 
My first thought too.
In the wild, your guide would have you well away if anything like that was going to happen.
Micros are iffy about zoos.
You may have to consider:
A vector or raster drawing
A bloke in a gorilla suit (as above)
A specialist macro agency, but you might not find it RF.