I know you said you didn't want to buy stuff, but... I find that grocery stores have the highest prices for clif bars/etc type products. Costco has big boxes of Larabars for a good price. Also, Trader Joe's has good cheap nuts, dried fruit, and other bars.

ETA: Also, if this is a supported century ride, I would imagine they'll supply food at the rest stops. Though, it can really vary and if you have specific dietary needs or preferences you'll be better off self-supporting.