Ask around your local older bike shops, they might have some old take-offs lying around in their back storage area. That's how I got my first set. If you're lucky they might have some better brand ones. The one I found at an LBS was a Chrous hubbed 32H Mavic GL330 set that according to the shop owner was exchanged for clinchers by the original owner after getting a flat a few miles into a first ride. It was really part of the owner's display of classic stuff around his shop. Fortunately, I was able to convince him the wheels will be better off on the road than hanging for many more years on his ceiling. After that first set, I'm now so hooked on tubulars that I'm about to build up my 4th tubular wheelset from NOS hubs and rims!
Chombi