Typewriter collector here. Ask me any question you might have.
Buying typewriters is risky business, especially online. It is always better to be able to pick one up in person and to take a look at it, that way you can convince yourself of its condition and mechanical shape.
Best would be flea markets I guess, but you will of cause be limited to the selection (or lack thereof) that is available there. Usually flea markets, especially in the states are quite rare on good condition, fully functional, mechanical (manual) typewriters, but there can be very good deals had. You will find plenty of more modern electronic daisy wheel machines in good condition there, but I doubt that's what you're after.
Another good option is craigslist or eBay classifieds. Look for something in your area that you can personally take a look at and pick up. There are a LOT of very good videos on how to use and test a typewriter on YouTube. The most risky part of buying online is shipping. People very often don't know how to properly ship such machines and they often arrive broken (happened to be more then just once).
Another way is to ask around your neighbourhood if someone might have a typewriter laying around, especially older people. You are not gonna pay much (or anything) and very often they are in very good condition.
First I would try to figure out what kind of machine you want. Desktop (office) or portable machines (in a case)? One you have decided on that, mechanical (manual) , electric or electronic? What brand, how old? what condition should it be in, what functions? I can help you out there if you want.
I would not try to recondition a machine myself if you don't heave any prior experience, you might end up using the wrong tools, lubricant and might make matters wore on the long term. It is much easier to just buy a functional one, some are very cheap to be had.