LAST EDITED ON 01-23-10 AT 03:44 PM (EST)I found it in a very large Asian supermarket. It appears to be a Thai thing only.
I have a bottle that I will never use up. Want me to mail you some?
ETA: I was thinking that Chinese 5 spice sauce would work in this, too. It's easy to find. And after I overdid the tamarind in my non-phone-it-in attempt to make this, I added ketchup to the leftovers and it was much better although a bit heavy on the sauce.
srsly