Pawn shops, flea markets, or shops reselling 2nd hand like Gamestop are your best bet for local shops. Online (mostly ebay, but also public announcement boards like craigslist) you'll have a easier time, but you'll have to cope with not knowing the product's state, or with highly fluctuating prices (since they're a collector's ware, by now, prices may vary greatly depending on the game's succesfulness and rarity).
I believe new products are almost surely out of the question, since they're out of production (hence why you only get them digitally, nowadays), although they obviously would be the best...
As a side note, when I talked about the need of a DOSbox and all those things of 16bit compatibility, I just wanted to make sure you knew what you were going towards by asking physical supports.
Some people can think you just have to pop in a disc and you're done, while it's not always like that; once again, sorry, but I had to make sure you knew that 😛