I got mine from CDNN Sports, but there's many places to buy them. Just google 'M14 handguard'. At CSNN don't expect a brand new black one, it's random what you'll get (black or brown, new or used), but they're good people to deal with. As far as I know the black ones aren't being made anymore, so it's more likely you'll get brown.
There's always Krylon.
They're fiberglass and extremely solid. They are designed to take the heat and stress of the real weapon after all.
I don't know about how they fit on TM, you shouldn't have any problems, just be prepared to do a little Dremel or filin