I know there's a lot of Pokémon Halloween costumes out there, but the likeliness that you'll find one of Sir Aaron is very slim. You'd have better luck going to a local costume shop (not a big retail store) and asking someone to commission one for you. If there aren't any shops nearby, you can commission costumes from some cosplay websites.
You could always try making it yourself too (if you or your parents have any background in sewing). It shouldn't be too hard to make; his outfit doesn't look too complex. The only thing that might be challenging is the wig.