The flea market in Sanford is slowly closing down shop. 4 of the retailers there have closed up shop.
There are Play-N-Trade stores in the area, but nothing with a large variety of stock. You may be able to find some liquidation stores in Titusville, but I'm not sure if they have physical storefronts besides their warehouses.
Besides that, in the downtown Orlando area, there's a place called "The Hobby Spot" off I-4 and Fairbanks. The guy has a decent amount of stuff, though his prices are fairly high. There's the Sanford Flea Market near Lake Mary Blvd and 17/92, though that's a bit out of the way...everything else is about 45 min - 1 hr away for decent places in the area.