That's sad. If you can get your snake to a herp vet, do so ASAP; I really don't think a non-exotic vet would be able to do anything except euthanize it if that's what you decide to do. Regardless of whether it's thiaminase poisoning or not, that sounds neurological and not like something the snake is likely to recover from. It's a tough choice, but it may be better to put it down.
It sounds like your snake was already going downhill when you got it, so who knows what happened to it in the petstore and before it got there.