smythge
If you have a good ole fashioned hardware store neer you, that has a huge selection of brass fittings,go there and rummage through them , if they will let you. This is what I do here in Burligton. I have allways been able to find
nipples and fittings that way. You might have to use reducers, or step upfittings but you can usually find something to work.
"Do not use Galvanized fittings" they cause corosion, and I don't think they are compatible with Anit-Freeze. If you have an aluminun manifold brass is the best.
The True Value Hdwr store here in B-ton has a huge selection of fittings and nipples.