first, your not going to be able to get r12 since they don't make it...maybe if someones been hiding a bottle at a shop but you will pay top dollar.
You will need to do a retrofit. it will mean changing your metering device(fixed orifice or txv) filter drier and changing the oil + new refrigerant.
I do not believe you can just drop 134a into your system. you might be able to drop in Hotshot with your current equipment. you will need to check with a shop on that part.
zac