Use of sour milk for bread making - how long after the expiration date is it safe?
I'm not so lucky as to be able to get whole raw milk…

Wait, then there's a flaw in your premise. Check the label. Any milk treated using high-heat processes like UHT, pasteurization or ultra-pasteurized milk doesn't sour like it used to in your grandmother's days; it spoils… goes rotten.
Spoiled milk is not the same as soured milk.
The ultra-pasturized milk sold in the supermarket is essentially a dead product, with little to no live bacteria. When it goes bad, it rots and should be thrown out (or you can make plastic with it). Modern industrial pasteurization kills the “good” bacteria that once made sour milk a wonderful thing for cooking and making bread and cheese.
Substituting &quot;Sour Milk&quot;
If you have a recipe calling for sour milk, put a teaspoon of vinegar in a cup of fresh milk and let it sit for a few minutes. That's about the best you can do if you do not have access to raw milk.