Luckily enough I'm in enough bands or on dep lists so something usually comes in.
We work with a few booking agents who always need someone at short notice.
I'm on stage right now in Bristol actually, but the gig only just came in. We were free so here we are.
Unfortunately my regular singer is so good she's in great demand so I have more free weekends than I might like.
Whoring myself out on Facebook sometimes works too.