Finding other musicians to play with is an easy matter once you realize that for the most part, musicians are always looking to make connections with more musicians. This means that wherever you can meet other musicians, you have a good chance of making a connection with someone you might be able to play with. Even musicians who are already members of an established group may be looking for an additional member or a second band opportunity (professional gigging musicians will often play with several groups concurrently), so strike up conversations with them between sets at local performances.
Another good idea is to put yourself in a position to be heard. If you feel comfortable with it and it's legal in your city, try a little street busking with your instrument. This can often lead to invitations and connections with other musicians as they pass by (and don't forget to make contact with a few street buskers yourself).
There is, of course, always the option of advertising and answering ads. Local online classified ads as well as those in newspapers often contain many such ads, as do community boards at local music stores. This is a good way to find lots of options for potential bandmates, though you may not have a chance to hear other musicians and assess their playing (unless they have a website or social networking profile with clips).
Once you've found some musicians to do some playing with, you're halfway to finding members for your band. Merely finding people who play the right instruments isn't enough, and even finding people who's playing you admire and with whom you get along with on a personal level won't necessarily ensure that you've found the right people to start a long-term musical collaboration with you.
In order to find your best band mates, you'll want to do some playing with the musicians you've met and see how you work together. Ideally, you want band mates who are your equals in talent, share similar or at least compatible styles and experience, have a similar vision for the band and its music, work well together and with you, and are willing to devote the same amount of time to the ensemble.
In order to start making assessments about these many characteristics, it's best to set up some playing opportunities with people you've met where there is an understanding that there is no pressure and no commitment. You may also call musicians together for what is understood to be a one-time project, such as a small gig, a freebie performance or a recording project. Otherwise, just make it known that you're asking a lot of musicians to get together for jam sessions and see what happens.