Does anyone know of someone or a shop (online / offline) that can turn a custom aluminum barrel for me?

Thank you.

Any machine shop in your area can do it. A local one I found in the yellow pages a few years ago did a 207mm barrel for a project I was working on at the time.

A good machine shop with a CNC lathe can do it but a one off project like that will be pretty expensive, esp. if you don't have a good CAD drawing or solid model.
Usually it is about as cheap to have them do 100 as it is to have them do one because most of the cost is in the set up time. You may consider, depending on the market, having them turn 100 and then selling them.

Just a couple of off hand Saturday afternoon thoughts,
Shaun

Heck, even my next door neighbor, who makes his own parts in his garage for the VW's he races, could have done it, if I'd known he had the lathe. He admonished me for not asking. At least I know for the next time.

Any one see this one (or tried it)
http://www.emachineshop.com/