There are two firms I use, both in the north of Glasgow. The more expensive option in North Canal Bank St is:
http://www.alloy-master.co.uk
They’ve got the professional set up and state of the art equipment, they’ll do your wheels as a same day service if you wish.
The second one is ESP Powder Coating in Stronend St, Possilpark. The owner Trevor is a friend of mine and although you won’t be particularly impressed by the premises, he does a first class job and will be cheaper than Alloy Master. He will do a same day service but will charge accordingly. He’s the best option if you can drop the wheels off and leave them for a couple of days.
I’d happily use either one, both do top class jobs. It depends on your circumstances really.