I love http://www.craigslist.org
for finding old sewing machines, but you can also find them in second hand shops and garage sales and sometimes even in a family member's closet :)
Look for a machine that doesn't have a lot of damage and always ask for the electric cord, foot pedal, owner's manual and all the accessories
. Sometimes you might not get all of these things but the foot pedal and electric cord can be hard to track down so I never buy a machine without them.
Ask to plug the machine in and make sure it works
. If you have to test it manually by turning the flywheel, remember to turn it toward you (counter clockwise).
It is nice to get a lot of accessories with your machine but you don't need them to learn with. Just the basics will work fine for most sewing.
You can bring some scraps of fabric and some thread with you to test the machine. If the owner has the machine threaded, keep the thread on so that you can see how it is supposed to be threaded (if there is no manual).
If a friend or family member is giving or selling you the machine, ask them for a few lessons!! Offer to do something for them if they will teach you :)
If you can watch video on the internet, this page has good information about the differences in sewing machines and what the parts are called: http://www.quilt-video.com/sewing-machine-tutorial/