Fortunately, I found a local manufacturer of polyethylene tanks, Custom Roto-Molding, Inc., so I was able to obtain these beauties without incurring enormous shipping charges.
The 310 gallon tall tank is for RO/DI storage, which will gravity feed the two 100 gallon tanks for new salt water mixing and storage. I'll plumb the two 100 gallon tanks together near their bottoms with a ball valve, so that I can empty one into the other, and use the empty tank to mix new water. Once the new water has settled, I can open the valve between the tanks to provide a single 200 gallon storage container for the automatic water change system to draw from.
Time to get plumbing!