There are lots of youtube videos which use jazz soundtrack. You could simply see what is on youtube in the genre you want? Youtube has some sort of agreement with the music publishers, I do not know what sort of agreement, but copyrighted music is not removed. There is also a mp3 download site called http://www.jazz-on-line.com which claims to publish public domain jazz (I haven't been able to verify any of their copyright assumptions, but many of the same track are available from archive.org in their digitized 78 single sections.) Many US army bands also publish jazz songs online, on youtube etc, and many of their things would be PD, provided you check the legal status of the composers and the artists.