My friend Susan has been poking me to post this, so this is for you Susan :-)

I created an Instructable on how to turn a plastic peanut butter jar into a saddle bag. It's the cheapest way to get a bag I've found.  I added as many photos as possible, I hope you guys find it useful!

http://www.instructables.com/id/Plastic-Jar-Bike-Saddle-Bag/