Online Ground School: This is the lowest cost ground school solution and is done on your own time at home. When completed recieve a certificate to take private written test.
One on One With CFI: Schedule ground school lessons directly with a CFI. This is more thorough and completed in addition to your flight schedule. Receive CFI endorsement for private written test.
Combo: Purchase the online ground school and brush up with a CFI on weak areas. When completed receive a certificate to take private written test.