It is still possible to buy complete engines although rebuilders don't like to do it, as they have only so many of the expensive, difficult to obtain parts available.  It's easier to get a brand new Renesis, since Mazda still makes the new parts.  You can even buy a factory-new crate engine of those.  If you know who you want to build your engine, then just call them and see if they'll sell you a whole one.

If you want to build it yourself, then just do some sleuthing.  Wrecked RX-8's are fairly common.  There's a guy I know outside Seattle with about a hundred RX's of all generations in his shopyard, going all the way back to the early 70s.  There are fewer and fewer 3rd generation RX-7 wrecks because there aren't too many of them left and the owners now tend to be careful with them, but sometimes they turn up.  rx7club.com has an active buy/sell/trade section.  For a core engine, it doesn't really matter what its history is.