You hit on the phrase that I think you should focus on. If it looks too good to be true, it probably is.
These unapproved substitutes are bad for your system and worst of all are explosive. If you ever have a compressor fail, and make a spark, you and your car will be an instant car bomb.
For the sake of you and your family I urge you not to use this refrigerant.
I share your frustration regarding the availability and price of R12. I have a few pounds of it left and when it's gone, it will be gone.
I have already converted some of my cars to r134. I worry most about my 124 car. I am saving the R12 for it. It appears that now that warm weather is approaching, the 124 a/c system will make one more year.
You can either use R12 at it's exhorbitant cost, or change over to 134 which is not too tough and will work well as long as you don't live in a Houston type climate.
Best of luck,