There are a couple, I think Aquarium Environments and Aquarium Design Group (one of our sponsors) do custom work. I'd recommend calling and getting some quotes, though, as I think they are geared more towards service than doing just custom tanks.
I ended up with an Oceanic custom, which was a little better price and I knew what I would be getting in terms of quality. I've heard of great things about Mircales in Glass, but you'd have to pay freight shipping for them (they're up north, but even with shipping it's competitively priced and they have great service). If you are on more of a budget, glasscages.com does a lot of work, but their customer service and ability to get an order right on the first try seems to be in question. Some folks here in town recently had a delivery from them, and they all seemed quite pleased.
Good luck shopping!