I'd camp around the buy sell section. *SOMEONE*'s gotta have a p90rds they'd wanna part with, sounds like you seem to favour that one more than the TR. (I know. i like my p90 rds dispite all the TR lovers out there.

tracking down a p90 rds metal receiver will be the hard part.. it took quite a while to get a hold of mine.
and yes if you want to mount anything other than a standard p90 rds onto the RDS receiver, you need to get a rail first.