Ok, so I'm pretty early on with Trevor. After dealing with a certain biker gang, I was hoping to nab one of the custom choppers some of them were riding. But It's like they vanished into thin air after the mission ended. And I haven't been able to find one since.
You will encounter plenty of custom bikes as you play the game. You will occasionally run into people who's been rob by a van, if you attack the van, bikers with custom choppers will show up to attack you.
Thanks! Yeah I actually finally found a diner last night with a few choppers in the lot. As I nabbed one, a few bikers ran out... a shootout ensued... and I ended up getting "wasted" by the cops. But I went back, and the bikes were still there. Got in stealth mode, and rode off with no problems :)