Usually, SSDs come with a drive bay adapter, which is pretty much a sleeve that allows you to mount a 2.5' drive in a 3.5' bay. Sometimes just a couple of brackets, sometimes a sleeve.
Either way, you could get some brackets online, something like http://www.amazon.com/SILVERSTONE-SDP08-3-5-2-5-Inch-Converter/dp/B002BH3Z8E