On the left side of your fueltank under the body (if it's still there..)
With some luck you can order a new one.
If you mean the general RPO code's, check the tech side of this forum on 81 vette's.
You can get a copy of the original build sheet and window sticker for your '81 (if it's a Bowling Green-built car) from the National Corvette Museum, and you can get a copy of the Dealer Wholesale Invoice (which has the same info on it) from a contractor to Chevrolet if it was built at either plant.