It's only available as a set, unless you can get them to split up a damaged set at the store. I doubt that they will though.
I have that same 26" set and love it. I put a HF 7-drawer side box on it, and plan on putting another on the other side to balance it out. Before I do that, I need to have a steel bottom plate made to keep the bottom box from folding under the weight. At first I thought that the castors were flexing, but it turns out that it was the thin bottom of the box. It's a good thing that I don't have to move the box very often.
Before anyone asks why I didn't just buy the 44" or 56" box, it is a money issue. I'm adding on as I can afford to. Besides, I really liked the drawer layout of the 26" set over the other boxes.