By the name pigeon peas, pretty much any Mexican market in the Bay Area. As mentioned, they are dried, though I think I might have seen some canned versions by Goya. However, just like any bean, go with the dried version. Where are you located? I can help you in the Oakland / Richmond area.
I live in Pacifica but I drove to the Mission District of San Francisco to a little latin american market called Lucas. They have practically any Latin American items but no gandules. I would be willing to drive to Oakland. I wonder what they taste like since they are hard to get. Thanks for your help.
Going in the opposite direction from Oakland, Phipps Ranch in Pescadero has them.
In terms of canned, turns out Goya and Pamplona are the most common brands. Here's a picture and an online source.
Phipps might be the highest quality local source. There's also that little Mexican Market in Half Moon Bay next to the fish store, though I couldn't swear they have them. But I'd look in the dried or canned bean section.
Phipps also sells online if you don't want to drive that way. I'm trying to think of some South SF Mexican markets that might carry them which might be nearer you.
It is just lately I've been into Central American beans and I've seen dried pigeon peas in almost every Richmond/San Pablo market I've been to. So I would think a drive up International Blvd in Oakland and stopping at some of the larger Mexican markets (especially those that note they sell Carribean groceries) might yield similar results.
I'm not going to be in the Oakland area until February, but if you don't find some locally, I'll keep an eye out when shopping and report back.