Check this old thread for starters:
http://www.guitartricks.com/forum/showthread.php?threadid=463

Also these sites may be helpful.

http://www2.one.net.au/~gighire/slide.html
http://www.slideguitar.com/index.html
http://www.slideguitar.com/guitstuff/slinks.htm
http://www.jodyfisher.com./