If you are looking for the physical drives, Your local Radio Shack, may have or at least obtain the drives, as well as the diskettes. They may even have the capability to install the drives, if you have enough available bays on your computer. I would recommend both, that is if you have a lot of data on the various media to convert or transfer, having one of each would make it easier. With the advent of cloud storage, I suspect internal hard drives and diskette drives (5.25 and 3.50 inch) will become obsolete.
ps: yard sales is also a good source and cheaper for computer parts and accessories.