I have no idea where to local one.

I searched on summitracing and they have an EARL radiator neck (http://www.summitracing.com/parts/EAR-166015ERL/) but for that price, I might as well buy a coolant reservior at $45

I would prefer a cap and a threaded neck so I can just weld the threaded neck to a tank I will built out of aluminum. Ideally, less then $15. Anyone know where to start.