> Any one dollar plain yogurt from the supermarket works fine
A surprising number use additives to thicken them, and as I result I found I got better results from ones labeled Greek or acidophilus. I have no idea if there is any science behind this claim, so YMMV.
Keep an eye out for faint orange "oil spills" on the surface or fizziness in the final product. Toss it if you encounter those and start over.