You can search and find who sells industrial gases in your area and buy the smallest bottle you can get. The deposit on the container will far exceed the cost of the contents.
You will then have to get fittings to adapt from bottle regulator to your manifold gauge set.
I totally understand your hesitancy to put even one ounce from your R12 cannister into a system that you are not sure is tight. I only have about five pounds left in my cannister and I'm scared to death of using it up. A 30 pound tank is now about $600.
Do you have a sniffer?