I found many of these stamps, including this one from Tuva, on the website of The Ebony Society of Philatelic Events and Reflections (ESPER). Its mission is to promote "the collecting of African Americans on stamps and the collecting of stamps by African Americans.” There you can view the close to 200 stamps on which King has appeared.
The Republic of Tuva is located in the Tannu Mountains on the Siberian border in northwestern Mongolia. It is famous for its throat singing tradition and its exotic stamps (can't wait to see these, and I promise to share if they are as great this set).
Dr. King often appears on one stamp in a series of notables, and sometimes with others on the same stamp. Then there are countries that have issued MLK stamps in various denominations. I’ve posted a small selection.
Sources: Aside from ESPER, these stamps are from eBay and various stamp-collecting sites.