With the explosion of the internet, fans now have an easy means to contact each other to exchange bootleg recordings. There are many U2 mailing lists which allow for the trade of bootlegs, and still other lists which are dedicated to the trading of bootlegs. Many fans also maintain webpages which list the material that they have for trade. Often you do not need to have recordings to trade to make use of these services, as fans often will offer to make copies for blank CDs and postage in return.
Traditional sources of bootlegs for sale still exist. If you are lucky, you may find bootlegs at your local record store. Mail order is another good source for finding recordings for sale, and there are several music stores on the internet that carry live imports