Things you can do about this:
* If you are a site host, check accessibility recommendations and accommodate whatever you can. Measure access ways and list things like door width, number of steps, etc. Make sure there is at least some seating for people who can't stand for long periods. If you charge extra for seating, reserve some at the usual price or better yet free for people with disabilities -- anyone with a car sticker should be able to get the reserved seating. Include contact information so people can ask questions. Use input from them to expand the information you list. Can't afford upgrades you want to make? Do a fundraiser for a bigger bathroom or a new ramp that won't ruin the historic look of your building.
* If you are a musician, insist that your venue hosts do the above. Do you do house concerts? Try to find fans with disabilities who would like to host some! Chances are, their homes will be accessible. Sometimes that is one heck of an asset: a house built from scratch for a wheelchair user has a minimum of walls, with spaces instead defined by columns or changing floor style, so that sound carries better.
* If you are a fan, encourage site hosts and musicians to make reasonable accommodations, whether or not you have a disability yourself. You might also boycott the ones who don't.