This is how it is listed. I agree, not all stores carry it. Because it is a base coat material, they may have an alternative. I would ask or see if you have a local masonry/tile store in your area.
USG Structo-Lite 50 lb. Basecoat Setting-Type Joint Compound
Model # 163841040
Store SKU # 122242
I used to dip paper towels in a plaster mix and then I would cover it with sculptamold. Now I just use the paper towels dipped in structolite.