If it's R134a the answer is yes.  For all other "acceptable for use" refrigerants the answer is No.  Only Section 609 certified technicians can purchase R12 or any of the alternative refrigerants, EXCEPT for R134a.  Anyone can purchase R134a, with or without certification.  The vendor who is selling the refrigerant must either ask to see your certification or have a copy of it on file.  If they do not they are in violation of the EPA's guidelines and are subject to fines and penalties under the Clean Air Act. 

Additionally, anyone can buy retrofit kits. Service technicians should keep in mind that there is no such thing as a universal retrofit procedure, or a simple kit a technician can purchase that will provide all the necessary parts to guarantee a successful retrofit for every make and model. Even within particular models, retrofit requirements may vary. A particular make, model and year vehicle driven for 90,000 miles in Houston may require a more extensive retrofit than the same make, model and year driven for 35,000 miles in Minneapolis.

The Society of Automotive Engineers (SAE) provides guidelines for a/c retrofit in their publication J1661. Several refrigerant and lubricant producers have published their own recommendations. EPA is compiling a list of organizations that offer either classroom or home-study (videotape/workbook) retrofit training. For EPA's current list, see "Resources for Retrofit Training" at the end of this document.