I went through the process of looking for a custom size aquarium, in the end I found the size I wanted as a stock item from cadlights. It wasn't a big tank, 70 gal, but I can share my experience with getting price quotes. I am ranking them from best to not so best quality below.
1. Reefsavvy: owners name is felix, does awesome work, glass is very thick which I personally liked, has a lot of options, but priced accordingly. Takes a while to get a quote as I think he is staying very busy.
2. Miracles: Almost went with them, price was just below reefsavvy. Reputation of very good quality, and responded to quotes and info very fast.
3. Cadlights: I know they do custom sizes, as well as some stock sizes. good quality, good prices. I didn't have any problems but I have heard their customer support is lacking. Due to the price, it was a stock tank, ended up buying their 70 artisan, just the tank no stand, and I am very happy.
4. CTC (coast to coast aquariums): got a quote from them. From what I heard they seem to be good quality, but I didn't dig too deep. they do runs specials occasionally as I have gotten emails, (i.e. free starfire upgrade)
5. I did get a quote from several local stores. They came in close if not higher than some of the makers on this list. I think they were getting them quoted somewhere in TX???
6. Glasscages: I found they had quite a few unhappy customers so I eliminated them from my list pretty fast.