To avoid blister beetle, try to buy hay baled before the blister beetles come out or after they are gone for the season. In my area, they come out after Memorial day, swarm around, feed, and lay their eggs by August, and they are usually gone after Labor Day. You will read a lot about beetle free areas, and about buying hay that hasn’t been run over with a conditioner, but honestly, unless you buy 35# bales from the Amish, it will ALL have been crimped with a conditioner and I really don’t believe people who say they don’t have beetles. I think even So. Cal. has beetles now, which leaves No. Cal. and Canada as sources w/o beetles (maybe ND and Montana).
Beetles are only active in the summer months, when they come out to swarm. If you buy hay put up before or after beetle season, you have the best chance of protecting yourself. I have started buying hay that is baled AFTER the first frost to make WAY SURE the bugs are dead. Alfalfa is a cool season grass and can make it past a few light frosts. Last cutting, in October around here, is usually super fine stemmed and just fine for my horses. This year I made a deal with a grower to buy an entire cutting out of one field so that he would put it up in small squares. Otherwise he puts it up in huge squares for cattle. Sometimes first cutting is kind of stemmy and weedy, and the horses waste a little too much, but I will buy it if I’m looking at running out before September.
I would find a grower who can intelligently tell you when his beetle season is and work from there.