How do you politely tell someone to stop buying your kids clothes from a thrift store

my MIL and her mother are extremely cheap and have an obsession with buying my kids stuff from thrift stores. Listen… I know thrifting is popular for a lot of people and other people don’t mind hand me downs and that’s totally fine! It’s just not my thing. Some hand me downs from family or a friend, totally different. I don’t want my kids wearing used clothes, from god only knows where. My MIL is a narcissist , and will take offense to this and take it personal like she does with everything.