I only purchase all of my corsets online.
I’ve purchased waist training corsets from Orchard Corset, Timeless Trends, and MystiC City Corsets.
I’ve heard that Swoon Busk, Crimson Rose Corsetry, and Dark Garden Corsetry have great waist training corsets as well.
It might even been worth while checking Ebay and Etsy. Just make sure they are made with real steel boning and have a money back guarantee.