Heavy cream plus cultured buttermilk. Look online -these instructions for example- for exact proportions, instructions on how to make. Buttermilk, btw, can be perpetuated by adding old cultured buttermilk to fresh whole milk (again, lots of online instructions).
I'd use real, cultured buttermilk vs simply adding lemon juice to milk. You'll have a better tasting, longer lasting product.
Probably doesn't make sense to make your own, except for the [much better] flavor. The only other kinda waste free option would be to find a salad/veg/potato bar at a market which has sour cream set out, or try and talk a counter person - where they use sour cream in a dish they prepare- into spooning some out for you … that, of course, is a stretch, but worth asking about.
Another possibility would be to experiment with vegan "sour cream" which can be made [for example] with cashews, salt, lemon juice and water. Again, do a search -you'll find recipes, variations.