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Could only suggest you have a look in local phone directory for a "powder coating" company, can't think of any other place that may do such without mega costings.
The process with it being aluminium is quite simple, although the finish is not just a surface treatment it is part of the casting material, i.e. the red goes all the way through the material. Yours would have to be either powder coated or anodised, personally I would go for anodised as this is actually a process where the colour is dyed onto the metal.
Go to www.alphaanodising.demon.co.uk for more info
Are the sti manifolds the same process as the Honda Vtec red top cam covers? Or is that just powder coating? I don't want a gloss finish, more of a matt one.
you could always tell your mates that you have just fitted an sti engine?! Unless he's just trying to repair the OE red coating on an STI, it tends to want to leave the manifold after a while.