Does anybody know where I can find some?
If you can make it to a farmers market, you're likely to find it. We just got some from Whistling Train farm at the W Sea farmers market last Sunday.
Foraged and Found Edibles has it reliably in the spring on Saturday at the U District Farmer's market.
I haven't done so lately, but in the past I've purchased sorrel in the herb section at Whole Foods. It may not be a better deal than the stuff you can get at the farmers markets, though.