I get mine from BRS.
You can go either PVC or ABS. PVC are stronger, but ABS has the advantage that if you torque them too much, they will break before your tank does. I've heard stories of pvc bulkheads cracking tanks. One issue with PVC is they generally require larger holes for a given size.
Regarding slip vs threaded, I would go with slip. Cemented slip joints are far easier and more secure, IMO. Many people worry about being able to take down their plumbing, but in truth, that rarely needs to be done, and when it does you usually need to re-do your plumbing anyway. New bulkheads are cheap, so expense isn't a big concern either.