It is actually quite easy to make your own skim milk rather than purchasing it from the store! It is a lengthy process, but if you are interested in trying it, here's the recipe:  

Step 1: Bring milk to a boil, keep boiling milk in a non-stick pot at a boil for 8-10 minutes 
Step 2: Cool for at least 2-3 hours
Step 3: Refrigerate for 10-12 hours 
Step 4: Remove cream from top
Repeat steps 1-4 a few more times to get skim milk.