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

Super geeky answer: Yak butter is impossible as yaks are the males of B. gunniens while females are called dri or nak. Wikipedia.

Having said that I do remember reading that ghee is very similar to yak butter, but is not, of course, truly yak butter. I would have no problem passing it off as yak butter if it was my pop-pop we're talking about.

This dude: http://www.king5.com/archive/64598782.html
is in Issaquah. Maybe he knows.

Are you sure this isn't a joke question?

Like a "smoke shifter" or a length of "shoreline" to tie up boats?