I have had a degree of success covering such events as a civic focus for local cable television; as a special documentary, focusing on production values and income from either the organizers or group of owners of the primary displays; as a guerrilla-style walk-on - shooting, conducting interviews and passing out business cards inviting those participating to contact and order; and by reading of such events in advance, approaching the primary organization for permission and releases, offering them a free production in exchange for purchase of some established minimum number of copies.
All have worked, and I rarely bump up against commercial competition (for some unknown reason) at these events, usually only having to deal with a group from the local cable or, if huge enough event, local television stations as well as the occasional CNN OCN or other. These guys really think they have the goods and can often become territorial dictators at times. I just grin and move on, or forward instead of butting heads or conducting pissing matches. They're going for the byte, I'm going for the bread.
You can run up against issues, so it isn't always advisable to try the guerrilla approach unless you feel really good about it. Best route is to get sanctioned by the organizers, a group of solid participants, or have some other inroad and access without stepping on the toes of sponsors, community leaders and local community cable, etc.