It's very unlikely you'd be able to find any of the supported players new anymore, and none of the newer players are currently supported, though some work is being done to try to get Rockbox running on the iPod Classic and similar generation iPods.
If you really want a new player to run Rockbox on you're kinda out of luck, your only option right now really would be to settle for a used player that is supported if you really want to use Rockbox.