Polyurethane are the recommended ones, .015 (tank ring) for the slim strikers, .016 for the fattys, probably 70-durometer. They are kinda pricey through McMaster at $8.61 per Pack of 10.
You can probably get away with using the buna instead of the polys, but they may wear faster. I get mine from the local bearing/oring supply shop, but you can probably find them at a plumbing supply/hardware store.
A good reference site: http://www.ottersccustoms.com/spyder.html