I don't know why it took me two months, but I finally clued in that I can make coconut milk here since I can buy fresh coconut. They're young coconut, not the brown ones, but I'm not going to follow the rule that says I'm supposed to use mature coconuts.
They sell coconut water and slices of coconut at the tianguis, so I just
chopped the coconut up and added some water, blended, and strained it.
Easy as can be, and it's a lot cheaper than canned coconut milk is
here. I can find coconut milk pretty easily, but it's about three
dollars for a can. This cost one dollar for a can's worth and since someone already dealt with the coconut and sliced it, I'm sold.