Many University Libraries have sheet music sections; check out level 7 of the Fisher library for a good starting selection.
Below are links to various sites offering free sheet music. If you're after a particular piece, try a search with the name in quotes, then boolean AND either "pdf", "gif", or "jpeg" to the search string. Note that some sites require plugins, use persistent cookies to limit the number of downloads, or are written in foreign languages (proficiency in French or Russian is sometimes of use).
Know any other good brass and wind sites? Contact the webmaster! We would be especially interested in any sites with music scored for Wind Orchestras.