The newsgroup is a good source for finding a tape tree, or distribution network, for a large number of copies of a good concert. Essentially it is an organized series of trades. Branches will make copies in exchange for blanks, but there is usually a good deal of side trading between branches and leaves. Tape trees can either be one-time offers or have permanent status. Permanent trees usually release two shows a month and are a good way to build a collection.
Many people initially get into tape trading by searching for a copy of a concert they attended.
You can improve your chances of obtaining the concert you want if you have something to offer in return. Many newsgroups post newbie-only offers by veteran traders who wish to distribute copies of a show without the hassle of setting up a tree. It's a good idea to read a newsgroup for a few days to get an idea of what's acceptable in the group before posting. One way to ensure being ignored or even flamed is to violate some basic tenet that would take seconds to learn. Once you connect with a trading partner, it's also a good idea to start slowly, two or four tapes at a time, to make sure your standards are similar and the trader is reliable.