Finding these machines in good working condition aren't exactly easy, and it may take some searching on the internet to really locate the one that best suites your tastes. First, you have to decide if you want an original or a replica. Originals are of course going to cost more, however, they do run the risk of failing to due the age of the machine. Alternatively, many high quality replicas are available.
To locate these replicas you can search on such sites as Amazon.com. In addition, I would highly recommend an internet auction site, such as eBay.com, or you can check in your local area on sites such as Craigslist.com. You will find a wide range of quality and price, so try to imagine your perfect vintage popcorn popper before starting your search to save yourself time.
It is important to realize that the cost of these machines can run anywhere from $50 into the thousands of dollars. The thousands of dollars would be aimed more toward the commercial market, whereas the less expensive machines are aimed at the home or consumer market. There are lots of different options for vintage popcorn poppers, and I would highly recommend getting a modern electric replica. IF you really wanted to go classic you could get a steam powered machine. The “original” vintage popcorn popper, created by Charles Cretors, was actually a steam powered machine. And these may still be had (though their ability to pop popcorn is considered inferior to today's electric models). It is recommended that the steam models be used mainly as a antique and the electric models be used for actual popcorn popping.
Some great inexpensive brands to look for would include: Atom, West Bend, Star, and Amana.