Junkyards, Mopar Swap meets, EBay, Mopar Performance Outlets and Car Dealers, some regular or specialty autoparts stores.

You can test a "used" one with a hand-operated vacuum-pump available at autoparts stores.

I believe it is considered a "normal wear" type part, so replacements should be (somewhat) commonly available.

I searched Summitracing, "Mopar Vacuum Advance Canister" and this came up:

Mallory (Mopar?) Vacuum Canister