I would highly recommend, for your own personal safety and the health of your a/c system, that you do some reading. You REALLY need to understand the fundamentals of refrigeration before doing this. You also need a manifold gauge set which is very inexpensive for R12 systems, about $30. Don't charge your system with a suicide hose.
As far as R12 at Pep Boys goes, there are MANY places where you can buy it as long as you have a 609 certificate.
A good source for reading is the MACS manual that can be had from www.ackits.com.