Ebay? :P You would need to go to audiologist first to get the hearing tests done and get the results to find a match of the correct hearing aid, they need to be adjusted for your needs... The cheapest way to get a hearing aid, if you're on low income with income limit cap. You can get them under a program for a few hundred dollars instead of few thousand dollars out of your pocket. Basically low income people get stuff for free while people with moderate or high income don't.
It's kinda like trying out someone's glasses that aren't right for your eyes.