There are, of course, numerous sources for buying sheet music. These links are for a few sources of (mostly) free sheet music. Because of copyright laws, sheet music for most modern jazz is not free.
Archive of Popular American Music
Almost 450,000 pieces of sheet music, anthologies, and arrangements for band and orchestra. The collection also includes 62,500 recordings on disc, tape, and cylinder.
List of links to other sheet music sites.
Mind for Music
"...the internet's largest collection of Jazz and Contemporary sheet music, arrangements and transcriptions, all completely free! On this site you can find everything from bebop solo transcriptions, to funk charts and big band music."
UCLA Sheet Music Consortium
"A group of libraries working toward the goal of building an open collection of digitized sheet music...a large proportion of the original sheets in participating collections has been digitized, allowing users direct access to the music itself and in many cases covers and advertisements that offer evidence of the cultural context in which the songs were published."