I got my old rims powder coated from ADA in Thomastown. I think they mainly do furniture and stuff like that.. but when I rang a few places around the Northern burbs they all recommended ADA. They did a top job on my rim. It costs $90 per rim, that's including sand blasting, acid dip and the powder coat.
$70 if the rims are alloy. They took about 5 - 6 working days, but that was between Christmas and Newyears. So they were flat out and understaffed (apparently)
Good luck!