You may already know your bandmates - you just don't know you know them. Some of your friends already in bands might be looking for a change, or they might know of other people who are looking for a gig. What makes this way of finding a bandmate good is that you "know the deal" about someone before they join the band. If so-and-so is a great bass player but has never turned up for a soundcheck on time in their life, now is the best time to find out. Finding band members through your local music scene grapevine means you can be more confident about finding someone reliable - or at least have a heads up.
The Record Shop
Where do music types like to gather? The record shop, of course! Or the instrument shop or another music related shop. Most of these places, especially the independently owned ones, will have message boards where you can advertise for band members. Be sure to put a little bit about your music on the advertisement, or at least some info about bands that you like, and don't forget to include some way for interested people to contact you. Asking the staff of these stores is another way to find people looking for a band.
Websites like Craigslist and MeetYourBand.com are filled with ads from people looking for a band to join. Browse through the lists of musicians in your area who are interested in finding a band with whom to play, and place your own ad seeking band members. Include the same information you would on an advertisement on the record store message board. You can also visit musician's forums, like our own to advertise.
Put a call in to the recording studios and rehearsal spaces in your area and let the people there know you are looking for musicians. See if they have a space where you can come by and post an advertisement for your band. The staff at these places know who is playing with which band and who is looking for a new gig, and they can be a great resource for you. Even if you haven't actually used the studio or the rehearsal space before, don't shy away from approaching them in your search for your band members.
Poster the Town
Stick advertisements up all over town, in places where you think likeminded musicians may frequent. Coffee shops, book shops, college campuses, clubs and venues - the key is to let people know you are looking. Get the word out there!
You may want to set up a special email account just for people to use to respond to your advertisements, especially if you plan on putting up ads all over in town in non-music related places, so you can avoid that guy who pulled your contact info off your ad on the coffee shop wall to try and sell you an insurance policy.
When you advertise, be as specific as possible about the kind of music you want to play. It's a waste of time for the jam band guy to show up at the metal band audition, and so on. You want people in your band with whom you share a lot of musical interests so you're more likely to have the same "vision".
Take the time to find band mates who can not only play, but that you genuinely like as well. Remember, if all goes well, you could be spending quite a lot of time with these people. No one likes spending 12 hours crammed in a van with someone they can't stand. Your shows will be better if everyone likes and respects each other.
Be clear about your expectations of a band member. How many practices a week do you plan to have? Will everyone get to have some creative input, or is there only one songwriter? It is very important to figure these things out in advance. Also, if you're serious about making a go of things with the band, let people know up front that you expect a certain amount of professionalism. Tell people you expect band members to get to the show or practice on time, well rehearsed, and ready to play.
If there are to be band related expenses, like traveling to shows, now is the time work out who pays for what.