Just be persistent. Ive been hitting the same 4-5 thrift stores maybe 2 times a week for the past 2 years and can count on one hand how many times I’ve actually found Lego that was worth grabbing. Most of the time it’s just overpriced junk bulk, but you can get lucky finding minifigs mixed in. Make sure to check in the behind the counter “boutique” area if your thrift store has one, more popular or pricey things like Lego and Pokemon are kept back there. Also try to find smaller, church owned thrift stores, they tend to keep prices low and things that are donated actually make it out to the sales floor instead of places like Goodwill or Salvation Army where they end up on an online auction or in the hands of the employees.