Making your own homemade whey is quite simple...
My method is as follows:
1. Fill a 1/2 gallon glass jar (rinsed out) with fresh raw milk (about 2 quarts), leaving enough room to add 2 TBS plain organic whole milk yogurt. Close with a tight fitting lid and shake gently to distribute the yogurt.
2. Let sit for 2-4 days until the milk separates into curds and whey (which looks yellowish). I found placing it on top of the fridge worked great. And 3 days is how long it took for me.
3. At this point you need a large bowl or pot and then I used a large strainer with 1/4 of a muslin type dish/tea towel laying inside. Cheese cloth may also work if the holes are small enough. Slowly pour the curds and whey into the strainer and let sit for several hours.
4. When the whey has stopped dripping (I would let it sit throughout the day) tie the ends of the tea towel to a wooden spoon to enable the rest of the liquid to drip out. Let sit several more hours or overnight.
5. Pour the clear/yellowish liquid into clean glass jars, label with the date and refrigerate. Now you have fresh homemade whey for your homemade formula and other lacto-fermented recipes!
The remaining curds can be refrigerated and eaten somewhat like cream cheese, but it has quite a tang to it which we did not prefer. But I have made delicious Rugelach with this from a recipe in Nourishing Tradtitions, by Sally Fallon.
Homemade whey lasts approx. 6 months and the "cream cheese" curds last about 3-4 weeks refrigerated.
2 quarts of raw milk make approx. 1 quart of homemade whey.