Autozone, Checker, Advance, etc.
As to whether or not they're worth buying, if you can afford wheels, they do add a little bit of style to the car in the meentime, but most people will say dont waste your money. I personally have had them on 2 of my 3 cars.
1999 Chevrolet Camaro SS (PC'd)
1994 Chevrolet Beretta Z26 Q4 (W41 M90'd)
2004 Chevrolet Silverado 2500 (4WD)
My wife's cars:
1971 Dodge Charger SE (stock and original 383 auto)
2004 Subaru Impreza WRX (kid-hauler)