Shopping around, I find plugs and caps in PVC, galvanized iron or black iron. Copper threaded caps seem nonexistent and brass ones are rare and expensive.
I don't want to use solder because it involves soldering.
I don't want to use brass or copper because the stuff is expensive. Can I get away with PVC? This is all on cold pipe. What's my best choice to avoid galvanic corrosion problems?