I have to tell you that unforunately there are very few featuring trombone solos. There are many sax and guitar transcriptions available free but most trombone players want to sell their transcriptions. I guess we trombonists have to be more entrepreneurial to exist.
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http://www.meister.u-net.com/dixie/hot_5_project.htm
http://www.jazzcenter.org/
http://digilander.libero.it/ricdis/transcre.html
http://www.saxsolos.com/
http://guitar.about.com/od/transcriptions/
http://pubcs.free.fr/jg/jazz_trumpet_transcriptions_jacques_gilbert_english.html
http://trumpet.voici.org/
http://www.jazztranscriptions.com/
http://www.stuntzner.brent.org/Transcriptions.html
http://www.lucaspickford.com/
http://www.transcriptions-pool.de/forum_index.php
http://0x68.com/jazz/leadsheets.asp
http://chnani.club.fr/ejma/ejrelev.html
http://www.shout.net/~jmh/terry/transcriptions.htm
http://www.iaje.org/article.asp?ArticleID=150 (Bob Brookmeyer solo)
http://www.alisdair.com/educator/transcriptions.html
http://www.jazztrumpetsolos.com/
http://www.shout.net/%7Ejmh/
http://www.alphalink.com.au/~michaelp/
http://www.geocities.com/jose_menezes/indexE.html
http://www.meister.u-net.com/dixie/louis_armstrong_transcriptions.htm
http://www.basinstreet.com/odjb/Page01.htm
http://www.basinstreet.com/odjb/Page01.htm
http://charlesmcneal.com/WebPatterns.htm
http://www.cannonball-adderley.com/part2.htm
http://www.geocities.com/andyw129/index.html
http://www.jeffellwood.com/transcriptions.html
http://www.encoremusic.com/trombone/1230258_alt_ge.html
The sites are a mixture of transcriptions, links to transcriptions, how to transcribe, and swap forums for transcriptions. Most of the transcriptions are free but there may be a couple that I included because they were too useful to ignore. Enjoy!