Your local auto parts store has access to plenty of them.

1978 D 100 manual disc, 1 and 1/8 bore
1988 Diplomat, 1 & 1/32 bore
1989 Dakota, 15/16 bore

Alot of "new" ones out there will be iron....most reman ones will be aluminum. Reman is fine if the job was done correctly.

Rockauto.com has some cool looking ones with a translucent reservoir instead of the usual black. I believe they're sold under the Dorman brand IIRC.