Talking about cheap ones, I highly recommend the extenders available on aliexpress that austfred first talked about a few years ago. They have the basic “proextender” for 20$, or vacuum extenders based on Phallosan Forte, an improved VacExtender with no vacuum ring… all for less than 50$.
I never tried ebay because there’s not much to choose from. I’m still using the china ProExtender that I bought 4 years ago, and it was roughly 100$ cheaper than any massively advertised extender.