If you have worked with a film composer before and you were happy with the result, the first thing would be to ask him/her again if he/she feels up to it and has time.
If – for whatever reason – you need to find a new film composer, there are several ways. Depending on what kind of size your project runs, you can directly contact composers whose music you have heard in other movies which you like. If you’re need someone more affordable, there are several ways. A good way is to ask colleagues whether they can recommend someone whom they trust. Also, using Google to find film composers is quite easy. There are thousands of aspiring composers out there. You might also want to try several music forums which there are plenty.
However, be aware that there might be quite a few wannabe pseudo professionals who have enough money to run an impressively looking website but are quite unable to produce good music. Whenever you start to seriously consider someone for the job, have a thorough “investigation” on him/her. Google his/her name, previous projects, check the IMDB credits etc. Even if you love the audio demos presented on the web page (which by the way every serious composer should have), it doesn’t mean that it is a pleasure to work with that person. Also, calling him/her and/or schedule a meeting will help to find out whether the chemistry is right between you.
When scheduling a meeting – as you are the one who’s in the need of the composer – you should make sure to start off the working relationship properly and find a way to finance any possible travel costs for the composer.