A few sites I have found interesting.
Please note that I cannot vouch for the quality of the sites or their products.
Sheet Music
General
Big Band
http://www.pdfjazzmusic.com/index.html
http://ourworld.compuserve.com/homepages/sheetmusic/arrange1.htm
Instruments and Equipment
Magnets for holding music on stand!!!
Maplin do some too - sizes may not be as good