Craigslist and classifieds are a place to start. If you don't know anyone in a new town/city and you're looking to start somewhere. These are great, but from my experience the personally gratifying, working, and paying bands I've been in came about from networking.
Usually it comes down friends of friends. You get known in your circle of friends (usually other musicians) as a bass player, you get asked to join a jam or meet up to play. Then you get known as a good bass player and soon enough the right band will call you.
It doesn't hurt to have a website, social media, youtube videos, etc. You can and will get gigs from the online "EPK" and it helps build a stable impression of professionalism. Yet, the networking circle is best.
My bands in Los Angeles I found/found me via CL. It's been fun, it's been a learning experience, most of all it's been a ton of work. It's been a good start. My bands in Arizona came about from networking and friends, although not as high profile as the L.A. bands I've had way more fun and made more money playing with the Az bands.