The one near me just lists itself as a regular charity shop. Not sure how you could search for one, perhaps google charity sorting centres? Smaller independent charities seem to keep their prices low. I never go to the ones in town as they're just so expensive for charity shops, may as well buy new. Depop, Vinted and eBay are also worth luck. Church sales are great! But I've only ever found them from seeing a sign outside. Visited one in lytham recently, I bought loads of real wool skirts costing just £3 each