I have bought from petco and told them straight up it was for my snake. No problem at all.
One time the guy went to get a rat out of the tank and one of them bit him. I said give me that bit you, I'll get rid of it for you and he did.
If a store PetCo or Pet Smart refused to sell me a rat. I would ask to speak to the manager, and if he refused, then I would ask for the number to the area director over the store.
I'm pretty sure it's not store policy to refuse selling a mouse or rat because your feeding it to a snake. Lets face it those stores are all about the... $$$