You don't need sponsors (and I don't need sponsors). BARF just said they would pick up the entry fee ($200 last year) if I wrote up my 2008 Cal24 adventure on their site and ran some stickers on the bike during the rally. Works for me!
The rally itself has sponsors that kick in product and $ to help keep it running each year, like Aerostich, Cycle Gear, etc. If you're interested in hearing more about it, and even considering giving it a shot, definitely check out
that site, especially the Past Cal24 Writeups section.