try fleabay for a used Compaq, Dell or IBM case....I'm not sure why you want the split back-plane...
Some of these servers have what you're looking for, and you can rip out the OEM mobo, and put yours in. Add a new PSU, and you should be all set. BTW, the mobo's on the older ones were non-standard size, so an ATX board may, or may not, fit. Get pics.