Pretty low chance. I've been breeding for about 3 years now, and I have seen a single mite on a newly acquired snake, once. I killed it and treated the quarantine room with PAM. That was all there was to it. I frequent a couple of pet stores in town, and I've never seen a mite in them. I haven't seen any at the local reptile show I go to.

Mites are usually associated with large wholesale facilities and with wild-caught animals. Even my WC garters did not have any mites (only capillaria, surprisingly enough--I would have expected more 'cooties'), though when I was much younger, I do remember seeing mites on wild snakes.
In a large facility that has many animals coming in and out, mites can transfer easily and be picked up and spread quickly. Bad pet stores are a source to watch out for, as they may buy from large wholesalers with shadier reputations. But anyone dealing with CH or WC animals may have mites get in.

If you find a mite (or mites) on a newly acquired animal that otherwise looks good, kill the mite and treat everything with PAM according to instructions. Inform the person you got it from--they will probably be grateful you said something, and they will need to treat their collection as well. If they're not grateful (and you were polite), you probably don't want to buy from them again. Mites can happen when animals are coming in and out.

In a closed collection, its extremely rare. It's unlikely that packaged beddings themselves will transfer snake mites (how would they get in there?), and very unlikely that any beddings from garden centers would either. (Beddings from garden centers may have harmless mites--but not SNAKE mites. The set of circumstances that would lead to snake mites in garden center wood chips is extraordinarily unlikely). More likely that single pregnant mites would be transferred when a snake in the store has them. Mites will crawl everywhere--onto bags, clothing, etc, so bringing home a bag of bedding from a store with mites may mean a mite hitches a ride on the outside, or in a fold of plastic. If you buy from a pet store, inspect their reptile tanks. If you see mites...tell them and leave. Be extra careful to brush off and put your clothes straight in the washer when you get home, and take a shower.

If you see mites at a show, promptly inform the people in charge of the show. Shows do not generally permit people to bring obviously ill or debilitated animals, or animals with obvious parasites, onto the premises. Whenever you see something wrong at a show, don't hesitate to inform the hosts. The hosts will thank you, and the other sellers will really thank you as well.

Mites are something to always keep an eye out for, but there's no reason to be ultra-paranoid.