Unfortunately I couldn't seem to track down yak butter anywhere, but if you're going to make this happen (and at this point I'd say it's a long shot), my best suggestion is to contact the folks at River Valley Ranch http://www.rivervalleycheese.com/
They're located in Fall City and have been known to sell yak milk mozzarella at Seattle farmers markets. You might be able to buy some yak milk from them, but I'm guessing you would have to churn it yourself...
Beyond that, it looks like if you want something that can be ordered online, pre-made, the best I could find is this Tibetan Yak Cheese -
http://www.kaboodle.com/reviews/tibetan-yak-cheese-8-ounce-by-igourmet.com
I used to order yak cheese soup a lot at the Himalayan Sherpa Restaurant on the Ave. I always wondered where they got the cheese for it. It was my favorite thing on the menu and could still be ordered for a while after it disappeared from the menu, but then unfortunately the restaurant disappeared too.
Or, one other idea is to go for water buffalo butter. It definitely doesn't have the same ring to it, and I can't say whether it's a good substitute for yak butter, but it came to mind after reading this quote from one of the River Valley Ranch owners - "The butterfat content of yak milk is very comparable to water buffalo milk" http://pnwcheese.typepad.com/cheese/2007/10/river-valley-ra.html
It seems to be available here:
http://gourmetlibrary.com/products/52121-Delitia-Buffalo-Milk-Butter