Great question, and I should have probably clarified my response. There are transformers, the "cans" you see on utility poles. Then there are transformers, the "big boys" in substations. This is one of the better pics I can find on Google, oddly enough it links to an article about how easy it is to disable a substation by shooting out the transformer: https://stopfossilfuels.org/electric-grid/shooting-transformers-disables-substations/buckskin-substation-transformer-shooting.jpg

The cans you see on poles are relatively ubiquitous and fairly easy to source. The big boys in the substations are the ones with the huge lead times and large cost. Stockpiling them as a business would be a loosing venture. It would be unlikely you would have a customer locally and transporting them is expensive owing to the weight/size.