Mikhail Bulgakov died in 1940 at the age of forty-eight.  In order to obtain an authentic signature or autograph, you will have to seek it in either signed copies of his published works, in antique shops, or in auctions which may included memorabilia.  I would expect you should save your money...unless you're lucky, this desire is apt to cost you a pretty penny.  Check out the links below for ideas on where to go and how to go about getting your autograph.  You should also check to see if there is an official site dedicated to this author, although, being Russian, it may not be in English.  At any rate, the contact info may lead you to a family member or friend who can get you what you want.  Good Luck!!