I live in south Seattle near Renton and there are a few places that sell them. McClendon's on Rainier (around $60) and Minter's Earlington Greenhouse & Nursery (around $50) www.mintersnursery.com. I got my tree, which I named Liz Lemon, for $35 at Holly Park Greenhouse & Nursery. It looked healthy and already had a few lemon buds growing. I was told by the seller that there's no such thing as dwarf lemon trees (which was what I was looking for), they can be pruned to become dwarf or left alone to grow into regular size. I'm not sure if mine is an Improved Meyer Lemon or not, if that's what you're looking for. The seller was very helpful in offering instructions on how to take care of the tree such as how often to water and fertilize. Hope this helps and good luck!
Holly Park Greenhouse:
http://www.yelp.com/biz/holly-park-greenhouse-and-nursery-seattle
Minter's Earlington Greenhouse:
http://www.yelp.com/biz/minters-greenhouse-and-nursery-seattle#atb_alias:AboutThisBizHistory/query:minter%27s%20earlington
McClendon's:
http://www.yelp.com/biz/mclendon-hardware-renton#query:mcclendon%27s