Developers are found on coding forums, artists are found on art forums, etc... but where to find someone who is good at making trailers? Is there some community of "trailer makers"? Should I look for someone in a game enthusiasts forum? Any hints where to look at? ...let me also mention that we have very low budget, and a volunteer would be even greater.
Are there any universities around your area? If so, consider hiring a student in an appropriate field. Many schools also have a classified ad site where you can post an offer.
Trailers are not very difficult to make, so not going with an established professional is a relatively small risk. These students will likely be looking to build a portfolio, so make sure you tell them that you're willing to recommend them if you're satisfied with the job.
Much of this will also depends on what, exactly, needs to be done. Do you already have the gameplay footage? Do you have the music? Is it just a matter of stitching clips together and adding music? Do you need a voiceover?
Being specific is helpful here. The absolute worst thing you can do is to just say "make a trailer for this game." Ideally, you already have a mental image of what the finished product looks like. Put these ideas down on paper in the form of a detailed descriptive document, complete with sketches.