You will require a quote from a Cisco authorised partner for your license, the SKU will be in the following form:
LIC-MS320-48xx-xYR
There are three models; non-PoE (simpoly remove the xx) Low Power (xx=LP) or Full Power (xx=FP) Then you have the choice of 1, 3 or 5 years licensing, for the last x (7 or 10 year licences available, but for a model that's already end of sale, that might not be sensible).
You must match the license to the model of switch hardware that you have; you can't for exampe, use a non-PoE license for a PoE switch.