I've just started getting a lot of gigs recently.
I don't have an agent, I manage myself for the most part. Directors normally hear about me through other people so I guess I get my gigs by having a lot of friends who play other instruments.
I only accept a few gigs now and then, so word of mouth seems to be fine for me and keeps me busy. You have to get out there and network, which I find to be the best way (if you aren't a full time musician). People are always happy to know anyone who is a live performer. You can also go to Student Life Offices of colleges/universities. They are usually always looking for performances.