I HAVE A SINGLE SPEED BIKE THAT I HAVE BOUGHT A NEW CRANK FOR AND NOW I NEED TO BUY A COG.

THE QUESTION IS
1.CAN I USE THE LOCKRING THATS WITH THE BIKE AND PRESENT LOCKRING OR DO i NEED A LOCKRING SPECIFIC TO THE COG.
2. I WAS LOOKING INTO BUYING A SURLY COG BUT I NEVER SEE ANY LOCKRINGS FOR SALE. ON E-BAY THAT IS.
3. I'M NOT USING THE BIKE AS A FIXED GEAR, I DON'T KNOW IF THAT MAKES ANY DIFFERENT

ANY HELP WOULD BE GREAT, SORRY FOR ANY ERRORS OR SPELLING MISTAKES OR WRONG TERMINOLOGY.