I agree with the folks above , Craiglist is a fabulous place to find local stuff .
I would think that Cardio on a treadmill would be much more benificial in the way your actually standing up and more muscles are being utilized arms are moving , legs are moving , torso is somewhat being twisted and motion versus a spin bike where you are sitting down and hands are on the bars not moving ,Although I could be way off because I do not condone the use of Cardio ,actually the only cardio I do is when im "on" cycle and I chase my Ol lady around the house trying to mount her everytime she stops and stands still like a horney fukn rabbit ....I mount , go 100 miles an hour finish and roll over ,,that gets my heart rate high , gets me sweating and some Ab workout in there to ,,,..Good luck on your search for your Cardio equipment ......