Just order Blocking grade skim milk from Sigma it is fairly cheap. Don't use any skim milk not intended for scientific work. By the way  you can use different proteins for blocking i.e. BSA.

I would agree to just buy scientific grade non fat milk.  I use the chem cruz blotto non fat dry milk from santa cruz and it works great.  The catalog number is sc-2325. Use 3-5% BSA if you are looking at phospho proteins because caseins in milk can reduce phospho signaling.

You might use 3% BSA instead to eliminate aroused suspicions. 

Good luck. 

Best,

Hamadi

Thank you very much, guys. All very useful responses.

Yes, it's better to use scientific grade SM but no, vitamins A and D will not influence your results in any way.