The Ronco Plastics catalog is online at
Ronco Plastics They make TOP quality tanks for a very reasonable price and have 400+ molds, 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. The online store here
sbo.com plumbing is now a dealer for Ronco Plastics and sells at a VERY nice discount. Go to the Ronco catalog to find the tank you need, print the drawing and then call Dave at sbo.com customer servce to order it.
In the unlikely event you can't find a Ronco tank that'll fit, Triple M Plastics
Triple M Plastic Products Inc is your best bet for one-off custom welded plastic tank.
You'll find more useful information in the discussions in the Head Mistress forum.