That sucks. I pop every snake I buy immediately after I receive it. Mistakes can happen. Definitely best to find out about it right away. I am not a fan of probes especially with small snakes. Popping is very easy with any sized BP once you get it good at it. I used to use probes and actually made a mistake myself back when I was newer to the hobby. I actually had a friend pic up a breedable sized het male of a high dollar morph from a show last season. After he shipped it down to me I popped it and it turned out to be a female. It was from a pretty well known breeder but luckily it worked out well for me. She has 7 follicles developing now.

Good luck with your situation. Hopefully you can find someone selling bigger het females for a decent price. If I were you I would look for someone that still had a few of last years hatchlings available and pick up at least two of them. With any luck you will be able to get at least one of them to go for next season and you will only be one season behind due to the sexing mistake.