What exactly was Cook's plan and from whom did he get the money?

Cook planned to double-cross the Driver by pursuing him after the robbery (which he did), killing him (which he failed at), and taking the money.

Driver learns this from Christina Hendricks.

From IMDB:

As Nino explains, the money belonged to an East coast mobster who was
moving it in to finance a new LA based operation. The pawn shop was a
front for him, and they would not want the police digging any deeper
into the crime than is necessary. Saying that there was only one
robber who ended up dead keeps them from tracing the money any
further.