So this isn't street food since it's just an ingredient, but the best place I've found to get jocoque is in the Monday and Friday tianguis from Chava's stand. He has lots of different dairy products that are good, but this is the only place I've found that sells jocoque from a bucket. I love it. On Monday is stand is the first dairy stand (next to the first fish stand) on the right if you're entering from the west end of the tianguis. On Friday he's the first dairy stand on the west end, next to one of the last produce stands. Or you can ask for Chava.
You can techinically buy jocoque in the grocery store, but that's boring.