Vicki - you did not say what part of the country you are from but I'm sure some members could recommend a reputable shop in your area.
Again, based on where you live you may or may not be happy with the conversion. If you live south of say, OHIO then you will not be happy with it during the heat of the summer
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I'm with Larry Bible - stick with the r12 - do not become one of the unhappy converts
. Knowing women (like I know them - LOL) and how they like their cool cars you will be better off just getting what you have in proper operating order. Do a search on r12 on here and read the results. As Steve B says - the r12 is probably the most inexpensive part of the mobile a/c system.