I usually get my microwaves for free. After all, they're trash. I certainly would not pay any money for them, on E-bay or anywhere else, whether they work or not.
If you're looking on E-bay and willing to spend money, you might as well try to find a suitable 3 phase transformer there. If only for the core if the voltages are not suitable for your application.
If I needed a 3 phase transformer, my first thought would be to visit the local scrapyards. Cheap & easy. Perhaps a machine dealer (who deals in 2nd hand industrial machines) could be a solution too. Usually they have a few old machine cabinets or control panels lying around from machines that couldn't be sold. There are bound to be some 3 phase transformers in there.
These 3 phase transformers may even be simpler to rewind than microwave transformers, which are usually welded. The industrial transformers may still have been designed and built to be able to be repaired, unlike their microwave counterparts.
Good luck,
Peter.
