************ LONG POST ALERT ***************
You've been warned.
If you just want more playing opportunities, and aren't particularly concerned about making money right now, make yourself available to churches for special music. Put together a mailer to send to ministers of music. Explain that you play tuba, and are looking for opportunities to play, and that you will provide your services without fee. Either enclose a CD recording of yourself or direct them to a website where they can hear your playing. Send your mailer to local churches, starting with "pretty big" churches (not the biggest, because they already probably have a tuba guy; not the smallest, because they probably never do anything where they can use you).
Some of these guys won't even open the envelope; but you may get a couple of invitations to play. These playing opportunities may or may not be musically rewarding; at first that doesn't even matter. You will have been heard by some local players. Be sure they see you behaving like an adult, dressed appropriately, and very disciplined and self-controlled. During breaks, let them know that you're looking for more playing opportunities. Have a simple business card with your name, "Tuba", phone number, and email address on it, ready to hand to anyone willing to take it.
Also, any all-city/all-region/all-area/all-state type band gatherings are extremely important for building relationships with other good players. If you hear a really good trumpet player warming up, and his t-shirt tells you he's from a nearby school, go over and talk with him. Tell him you noticed his playing, and hoped he might be interested in something you and your trombone player friend are doing. See if he knows a french horn player. Or a good clarinetist.
If you do play in any sort of all-whatever band, try to make a good impression (as both a musician and a mature, polite young man) on the director. My youngest, a 15-year-old trombone player, played in the Knox County (TN) Honor Band last week. The director happened to be my director in the East Tennesse Concert Band, a very good community band. He had never met my son, though, so he was unaware of any link to me. But my son played well and behaved well, and the director was sufficiently taken with my son's playing and demeanor that he invited him to join the virtually all-adult band (in a trombone section not particularly short on trombones, I might add). This director is also very well connected (he's a retired director of bands from a local university), so a referral from him in a couple of years may even help my son win some scholarship money. Of particular note: my son was NOT the best trombone player there, though he is playing very well these days. But in addition to good solid musical skills in a variety of styles, he showed poise, the right balance of confidence and humility, good manners, and a willing attitude. Anyone can learn to triple-tongue; some people never learn to be gracious.
In short, take advantage of every opportunity to connect with other musicians, and conduct yourself in the most professional and mature manner possible when you do. Learn to play another instrument, or even percussion

. Never turn down any opportunity to play, even if it doesn't sound like fun. Learn to play chord changes. Buy a fake book. Sing! And be really, really NICE to everyone! Oh, and follow Rick Denney's first law of playing: first, do no harm. If you can't play that very exposed embellishment, leave it out! Even if someone notices it missing (unlikely, really) that's better than EVERYONE noticing it messed up!
BTW, once you're a little more established, you can send out another mailer to those churches, reminding them of your availability but letting them know that, due to the demands on your time, you are no longer able to play without compensation (
if you want; for me, playing in church is purely an act of worship. I make my living elsewhere, so if I ever am payed by a church, I endorse the check and drop it in the offering plate. But I'm not suggesting that everyone else should do that, and certainly not suggesting professionals should do so).
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Joe Baker, who went through a similar process when he moved to a new area a couple years ago.