Locally your best bet will be plumbing supply warehouses. In bigger cities, they're common. Forget it in smaller towns (<100K-ish). Online,
www.mcmaster.com has anything and everything you could want. Shipping is reasonable unless you only want a couple of fittings.
You'll be hard pressed to find nipples with extremely short thread lengths as they're simply not standard. Gotta do a little cutting on your own for those.