Having built a few bikes myself and searched for alloy tanks I can tell you that you don't get what you don't pay for. Inexpensive alloy tanks usually have a reason that they are inexpensive. I bought a tank from Unity Equipe some years ago and the aluminum was so thin that it wrinkled while sitting on the shelf (heat expansion?).
Expect to pay 400-500 pounds or $800 to $1200 for a quality tank. The tank shop offers good tanks, but they are pricey:
http://www.thetankshop.com/
Andy Cowell also makes custom tanks, but he is on the upper side of the price I quoted and reportedly takes a year or so to deliver. Andy can be reached through AHRMA.