As an ABYC member I tried to build my tank to the letter of the law/standard. This meant no mixing of copper alloys and the aluminum tank as many manufacturers did and still do.
So, where the heck do I find aluminum tank fittings? I asked a friend at Morris Yachts where they got their fittings and he sent me to Luther's Welding in Bristol, RI 401-253-5550. Sure enough they had aluminum tank fittings, and shipped out exactly what I ordered. The prices were also very reasonable. Great folks to deal with.