I recently purchased a conversion kit from Walmart (in California and should be offered in most states).
It offers three cans of 134a, adaptors, gauge w/hoses, instructions and a DVD/CD with step by step instructions.
The conversion was simple.
Go to (most any) service shops and ask them to evacuate your R-12 freon (most will do this for free, as R-12 is almost gold to them). Then attach the adaptors and charge your system to the required amount of 134a.
I have a very cold Jeep now.
Keep Jeepin and chillin