NFL's Chris Johnson -- $5,000 Reward for Shooter's Arrest

The person who shot NFL running back Chris Johnson -- and killed his friend -- now has a $5,000 bounty on his (or her) head ... TMZ Sports has learned.

Law enforcement sources tell us ... investigators have been working like hell trying to identify and locate whoever fired the shots in that fatal drive-by shooting in Tampa on March 8th. Johnson was hit in the shoulder. His pal "Rollie" was struck in the head and died from his injuries.

Sources tell us ... cops have received several tips -- but so far, no arrests have been made.

We're told cops are hoping a $5,000 reward -- offered through the Central Florida Crimeline -- will motivate people to share information that will lead to the killer's arrest.

The cash was raised through donations -- and tipsters can be as anonymous as they want.