Best place to get the plugs needed to replace one of these elbows

Try Jerry @ Stainless, the plugs you need have Anodes on them to help with electrolysis.

Brass pipe plug from the hardware store

You have Aluminum manifolds bolted to iron heads and stainless risers, I personally would go with the ones with anodes. Your boat, your money.

Agree, local hardware store

No issue with Aluminum to Brass (elbow)

No issue with Stainless to Aluminum (also has gasket)

Is an issue with Cast iron to Aluminum but there is a gasket between the two

Like Griff said , I use a brass plug , or you wanna get fancy get a 3/4" anode and cut the anode short if it`s salt water .. I did that too and on my intercoolers

He's replacing a regular 90* brass fitting, so I don't any issue using a regular brass pipe plug.

mcmaster carr is a good source for hardware.