If you haven't already, you might try searching your entire hard drive for files with "m4p" in the name. Who knows, maybe that'll turn them up somewhere.
Barring that, my only advice is to make backups of all your purchased music. I haven't had this happen to me but I always copy a purchased song to several other computers immediately after downloading it. Eventually they get burned to a CD or DVD as part of a routine backup, too.
Also don't forget to periodically backup the folder "/Users/Shared/SC Info". This is where iTunes authorization info is stored.
Hope this helps!