Here is a quick render of the jet model that I put into Sea Assault. I've gotten better at using Cheetah3D over the years. In this case, I had to turn the subdivision down and set the smooth angle high. Often, the biggest performance boosts have nothing to do with programming but are simply trying to reduce the vert count on models as much as possible.
Newer versions of my game engine ( toolbox ) use vbos. But since the iPhone/IPod use main memory, it does not make as much of a performance boost as it would with a video card running on a desktop. Though, since new hardware might come along and give benefit to vbos its wiser to make use of it anyways.
If a model is really a unique recognizable item, sometimes I'll buy it on Turbosquid. But its difficult to find low poly models sometimes. The jet is really generic and I could get away creating it myself. For Sea Assault, I did all the model work and art myself.