I just took mine out the other week while replacing a broken leaf spring mounting bracket. Often times (especially if you live in an area that uses lots of salt) the bolt that goes through the bushing will be siezed to the metal insert in the center of the bushing. In order to get it out, I needed to but the ends off of the bolt. Next, I drilled several holes into rubber the entire way around the bushing until the bushing could be removed (using needle nosed pliers you will be able to start pulling pieces out untill it all just pulls out).
As for new bushings, I used replacement bushings availble at Napa, Advance Auto etc. They have a steel outer shell with rubber inside that and a steel insert in the center which holds the bolt. The only way that I was able to insert the new bushing was to remove the entire leaf spring and press the bushing into the leaf spring using a hydraulic press.