Thanks for all your help. No luck yet.
I think calling xni-pec is probably my next step.
I've not checked at the Maxwell mkt yet, but if i can get there, i'll try to find it (and ask vendors).
Chaya is a distinctive, omnipresent green in western Yucatan. I've had it in everything from agua frescas to tamales to soups to desserts to health drinks at the beautiful Hare Krishna restauarant in Merida.
Swiss Chard is probably the best substitute, but there is a weird, almost allspicy flavor in the Chaya that i like. It's not quite as sharp a flavor as most dark greens, more spicy.
I'll re-post if i find any (and if i make anything out of it!)
Paul