The first thing you need to know is the operating voltage and current consumption of the radio on full output transmit.....so how do you find that out?

Hopefully you will have the instructions, operating manual, etc that came with the radio....it should tell you in it's specifications section. If you don't have the "specs", then do a search on the internet using the brand and model number of the radio and the word "operating manual", "manual", "users manual", "specifications", etc.

Look for voltage requirements and transmitter current, etc.

Most standard sized mobile radios require 12 to 15 volts DC. If the transmitter "pulls" 10 amps on full output transmit, then look for an external power supply that will handle AT LEAST that amount as a continuous rating....AT 12 TO 15 VOLTS DC.

It is a good idea to get a power supply with 25 to 50% more output than is needed by the radio so as not to overload it. It must be fully regulated meaning that the DC power coming out of it is very "pure" with very little voltage changes under full load. Do not use a power supply that is not regulated. It may produce very "dirty" rf and may harm the internal circuits of the radio, so trying to save some money will cost dearly in the long run! Just make certain the power supply "specs" can be matched up with the "specs" of the radio requirements. If it seems like a bargain price, then maybe you should do more research....stay away from "too good to be true" pricing unless you have a more experience ham to help you make the decision.