Where can I find yak butter online or in NYC?
Often finding exotic ingredients means determining which cultures use that ingredient, determining where there's a high concentration of immigrants from those regions, and checking for stores in that area.

In this particular case, we know that yak is a staple of Nepalese cuisine, and searching for Nepalese restaurants in New York City, we find a cluster of about 8 near the intersection of Roosevelt Ave &amp; 74th St.  There's a smaller cluster of three on Queens Blvd. between 39th Pl &amp; 50th St.

... and just searching for 'nepalese groceries nyc' finds Himalayan Mini Mart, which is sort of between those two clusters (39th Ave &amp; 64th St.)  There was also an article on the growing 'Little Nepal' in Jackson Heights.  Following a few leads in that article, I also found a Himalaya Bazaar closer to that larger cluster of restaurants.