People don’t just donate Halloween-worthy goods during Halloween time, you know. Behind closed doors, local thrift stores and better-known thrift store “chains” (looking at Goodwill, here) have all kinds of Halloween stuff - decorations, costumes, you name it.
Most of these stores will sit on Halloween-related inventory through all of winter, spring, and summer, but then it all gets taken out and set up during the Sept/Oct timeframe. Instead of recycling the more “funky” items, some stores will hang on to these items and add that to new/inexpensive costumes that they purchase for resale.
Bottom line? This is a great time of year to find vintage western wear (boots, pearl snap shirts), old bellbottoms and vintage 1970s, vintage uniforms, “hippie/bohemian” style items, and military clothing - not to mention that many of the military items will still have their patches and medals affixed. Naturally, this is a big draw to a lot of people, so don’t think that some of the cooler items will be hanging around through the end of October.