Look into local community college or general community bands. The repertoire can vary from medium level high school to college level music. Watch the local events section in your paper and go check out those types of concerts to get a feel for what they play. Most bands these days have at least a basic website.
Do a Google search for "community band" + cities in your area and you're bound to find something if it's out there. Some require audition, some don't. It's generally required to talk with at least the conductor or personnel manager before showing up to rehearsals.
Generally, the groups are comprised of adults and the occasional high-schooler.
You might find the experience to be fulfilling, really challenging, or really damned frustrating. The group I'm in is all of the above at times, as I'm sure most bands are.
There are some bands that think they are better than they really are (more attitude than talent) and there are those that are awesome and don't make a big deal about themselves. You never know until you hear them or play with them.
To play in a band that is "better than you" can be a great learning experience if you are a good sport and work at it. To play in a band that is "beneath you" (skill-wise) can also be an opportunity to contribute and grow as an individual.
Obviously, I'm an advocate for community bands...