The major sources of recycled R-12 are scrap yards. Recyclers would go around the country to find pre-R-134 scrapped cars.
In these case , they would pay the scrap yard owner a certain amount for each car they suck dry of R-12
Some re-cyclers would go the distance to clean and filter the old r-12 before reselling it. others will simply sell the stuff as is with all kinds of contaminants in it.
Buying re-cycled R-12 is somewhat like playing roulette. You are never assured of the quality of the product you are putting in your system. Serious shops would be safer than the backyard ''Joe Knows All" outfit