In my town almost any refrigeration place has it. Its not cheaper, actually had been for some time, than R134a. Its legal to put in a system that was made with it, just can not use on any new systems. I would just call around a few refrigeration places and see if they can load it up in a can or even the car itself.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car