http://autorefrigerants.com/co00033.htm
This link will get you to where you can purchase direct R 12 equivilant drop-ins, without the need to flush your system. I would recommend full evacuation by a HFC recycler, and do a good one hour evacuation before you add their oil and substitute refrigerant. If you can get R12, thats fine, but they are getting up to $60.00 bucks for those little cans. I remember the days when Strauss Stores sold a can of R12 with a filler kit, for $3.99.....