Hi, gizmo, I'm from Houston, and I wish I could tell you that you can find cheap cubes here, but I can't guarantee it.
You've got a couple of possibilities: 1) Bed, Bath and Beyond here carries them this time of year when all the college students are going back to school, and may still have some left. 2) Walmart, and maybe Target, list them in their online catalogs, and provide free shipping to the store. You have to pick them up at the store.
When I was looking a couple of years ago, I gave up on the cubes and bought closet shelving at Lowe's or Home Depot -- don't remember which. It wasn't any more expensive than the cubes, but you don't have the flexibility of changing your cage around like you do with cubes. You pretty much have to plan it out like you want it before you even start. Shelving is, however, sturdier than cubes, so if you want to make multiple levels, or have other animals to keep out of the cage, it's a good choice.
Welcome to the forum!