oem navigation dvd

Hello there im new at this forum... I need help i bought a used navigation unit... I want to install it in my 2008 silverado c/c w/o the bose system... I'm aware i need the gps antenna and a map disc.. My question is.. Does anybody know if any map disc would work on the navigation unit because i see that they sell some on ebay real cheap or would i have to go thru the gmnavdisc website to get one?

Thanks for your comments in advance....