There's a gun shop local to me that will have pre-bans, and he'll advertise them as such, but people around here (MO) seriously, don't know what "pre-ban" is, and don't really care, so they don't command the price that they would there. So, you'll find nice Colt Sporters for $1,200 or so. With FB and the various pages one can find, I'd think it would be fairly easy to find what you want so long as you have a competent FFL that will receive it for you.
I keep an eye out for preban stuff and have directed a lot of said items up that way.