I have been purchasing flashes off of eBay for about a month, trying to built up four or five manual flashes for some upcoming school work. Cost is a major issue, so I have turned my attention to older, used, film based flashes such as the Vivitar 285. If you are patient, you can pickup a working 285 for under $30, and if you are lucky, possibly even cheaper.
One of the big concerns using older film flashes is the trigger voltage. You will need to check your DLSR manual/vendor website and determine how much voltage your camera can handle at the hot-shoe compared it to the flashes trigger voltage (most modern DSLR’s require v<=6V).
A great site for determining the flashes voltage is http://www.botzilla.com/photo/strobeVolts.html but I strongly suggest that you learn how to check the voltage yourself. Some flashes can very greatly in voltage depending on manufacturing date. Fortunately, Jeff Geerling has written a voltage checking tutorial for the Vivitar 285 at http://www.lifeisaprayer.com/articles/photography/measure-voltage-vivitar-flash
Even if you know the voltage it can be a little scary mounting and using the used flash on you high dollar camera. I also suggest protecting your equipment with either a Hot-Shoe Safe Sync, or triggering with remotes. You will still need to check the voltage on both the flash and the remote, but you get the added benefit of a remote trigger and an air-gap between your DSLR and the flash. If worse comes to worse, you fry the trigger, not your camera.