Well, both Rhapsody Developer Release (Rhapsody 5.0) and Rhapsody Developer Release 2 (Rhapsody 5.1) are very rare, but pop up on ebay from time to time. If you are just wanting to "see" what it was like, I guess either would work. If you want to actually "use" Rhapsody as a primary OS, Rhapsody 5.1 was pretty close to the finish product (I use this as a daily OS on my ThinkPad). On the other hand, if you have a spare Mac, it is way easier to find Rhapsody 5.3-5.6 (Mac OS X Server 1.0-1.2v3) for around $35 on ebay. Plus there was way more software for the PowerPC version. I currently have Rhapsody 5.6 running on a PowerMac 7500 with a PowerPC 604e/225 upgrade.
If you find Rhapsody, a good place for getting started using it is my
Rhapsody Resource Page. And I may be able to help point you in the right direction on things if you want to PM me.
As for NEXTSTEP, You can find 3.x on ebay quite often, but if you can't wait I would recommend
Black Hole. If you are (again) just looking to "see" it running, either 3.2 or 3.3 would do. If you are wanting to be productive, I would suggest at least 3.3 with Apple's
Patch 3 (includes the Y2K patch) and would highly recommend OPENSTEP 4.2 (with
Patch 4) as it has more up to date software available for it.