I think this may be the main event. I haven't visited this guy yet but I really want to.

Hillside Desert Botanic Garden

Looks like a terrific display.

I am most familiar with the retail nurseries in the area (my customers :-)) and while there are several very nice ones around Yakima, there are none I would intentionally seek out for unusual selections - they carry pretty much the same stuff retail nurseries and garden centers do here, although a somewhat narrower palette because of hardiness issues.

Ian's suggestion looks like a great place to visit! I've got a sales trip scheduled for that area in a couple of weeks and would love to squeeze a visit in, although hard to do when you have a rather strict itinerary to follow. And there's a couple of wineries in the Walla Walla area that are on my list as well :-)) I've not been yet, but the Yakima Area Arboretum may be worth a visit. It does have an area that specializes in plants native to eastern WA, as well as 46 acres of planted grounds.

Here is a link that might be useful: Yakima Area Arboretum

I went to the Yakima area arboretum in winter 2001 (after a failed snowboarding trip to White Pass, which was closed because of high winds). At the time it seemed only moderately interesting, with large undeveloped areas and not a huge variety of plants. Of course that was a long time ago and it was the middle of winter, so perhaps things have changed.

Definitely worth a stop, some nice specimens there. Was started in the 1970s.

The above cactus collection and related content are featured in the current WSU Master Gardener magazine. See the magazine's web site.