i'm not sure where to buy one, but i'm sure you can search for shops online, i've heard of some chinese stores that sell them :)

If you’re looking for pre-made or made-to-order, I’ve seen a lot of Roy Mustang costumes on ebay so you might want to try there.

Not sure if made-to-order costumes on ebay are reliable or not but give it a shot if you don't plan to make your own.

1. Get is commissioned.

2. Do what i did. Make it yourself.

It doesn't really matter which military uniform you get on eBay; they're all the same. But you'll also need Roy's fire gloves (And maybe dye for your hair to make it black if it's not already).
You can find them in all sizes on eBay, or you can search Froogle (Which can be found at Google.com)

You could waste a pile of money commissioning one, or buying one off eBay, like everyone said.

But you'll probably get the best fit and best results if you just make it yourself. If you have no sewing skills, just find a friend/family member that'll help you out. Even still, a lot of sewing shops offer how-to sewing workshops where you can ask for advice.

And it helps a lot if you have/know where to use a sewing machine.

Seconding the eBay idea if you can't sew yourself.

I found this place they have a lot of good military outfits. And they have different styles that you can choose from. Also they come with Mustang gloves and the hat.
http://cgi.ebay.com/FullMetal-Alchemist-Cosplay-Roy-Mustang-Army-UniformSet_W0QQitemZ6029340862QQcategoryZ106892QQcmdZViewItem

qqcosplay.com is pretty good. Their prices are very reasonable. :)