I don't know how powerful you want. Craigslist or ebay might be good places to look to purchase one.
If there is a university near you, you may be able to make some contacts there and see if you could get some time on their microscopes. You may have to be a little covert with that approach though. I don't know if a dept. head would give an outsider access to, say, a scanning electron microscope......doubtful.
Another thought would be, if you have a tech school near you, you may be able to sign up for a night class that involves the use of microscopes. That would be a relatively inexpensive way to gain access.
One more idea. Occasionally universities will sell off old equipment dirt cheap, you could give them a call and find out if they ever have these sales. I go to these sometimes. My last big score was a big fireproof safe, in used but good condition, that I got for $1.00. They gave me the combination too.
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Those who would give up essential Liberty, to purchase a little temporary Safety, deserve neither Liberty nor Safety. Ben Franklin
The night class idea is very good, for more than one reason: you'll also learn how to use the microscope. There's a little ritual involved in getting an image at high power, so unless previously trained, the training is really the first place to start. You have to find what you're looking for at the lowest res, then move up step by step and zero in on it.
I got a microscope from a friend who used to go to auctions and buy up gear that was being unloaded probably from some technical college after an upgrade. It doesn't work entirely but luckily I can work around it and still get good images on the lenses that work.
As for prices, you can't beat internet shopping. Just google microscopes for sale..