Hope you are enjoying this fabulous late Spring weather as we are here in the wheatfields of Eastern WA. We're in the early stages of a "full Monty" on a '58 FC and I thought I'd share my experience in obtaining the black and gray tanks. After a great deal of research and online shopping I settled on an outfit called Pelland in CA at
RV Parts and Motorhome Parts Online Outlet
They had a huge catalog and reasonable prices. So I placed an order back on March the 8th. To make a long story short, UPS finally dropped them at my door today, May 15th. Made of thick black poly they seem like they'll be Ok, but here's the interesting part, they were shipped directly from the manufacturer which is an outfit in Idaho called Custom Roto-Molding Inc. I googled them and sure enough they too have a nice website with a big selection and sell retail. I didn't compare their prices, but it seems to me they'd probably be a little cheaper and surely a little quicker since you're eliminating the middle-man.
Custom Roto-Molding, Inc. - Rotational Plastic Molding - Oregon and Idaho