Your best source is Ronco Plastics
Your original tank was almost certainly aluminum...replaced after urine had eaten through it, hopefully with a plastic tank, but most likely an "off the shelf" rectangular tank.
Ronco Plastics (no relation to Ron Popeil and his Pocket VegoMatic)
Ronco Plastics makes TOP quality plastic tanks for a very reasonable price--which becomes even more reasonable if you order it through the online store on this site--and has more than 400 shapes and sizes, over 100 of which are non rectangular. And they install fittings in the sizes and locations specified by the customer when they make the tank.
If your existing tank is more than about 6' from the toilet, you'll want to find a better location for it...I'll be glad to help you do that. Meanwhile, you might want to spend some time browsing the discussions in the Headmistress forums for some insight into how to install and maintain your system.