Hello, I have a MTB that I use for city commuting. On warmer days I wish I had a rear pannier so that I didn't have to carry things on my back. My frame doesn't have the necessary 'holes/eyelets' to fix a pannier to. The rear fork has an eyelet on one side, but not the other.
Try one of the bike racks that clamp onto your seat post. They aren't real strong but you're not going to carry anything real heavy, are you? You might try taking the bike to your LBS and asking them what they would suggest.