I've got American/something else hybrid chestnuts that have naturalized in a small area of SW Washington near the Columbia. they're larger than pure American, but sweeter and easier to peel than Asian or European species. and I've got a couple varieties of groundnut that seem to be doing well here. Hudson and Densituberous, I think. both have set seed, though I've never collected any. you're welcome to some of any of those if you stop by, though I don't know how well they would make the transition to your side of the state. I'm also looking for skirret for myself.
I have been looking for groundnut for quite some time. It seems there are a few online places that sell it, mostly in the US, but they only have limited supply and only sell them by December, during a short time. I think the best would be to wait and order from http://www.oikostreecrops.com/
Chinese yam; its available in ebay as mentioned. For me, the difficult to get is air potatoes.
Skirret: I got a plant 3 years ago, which I am growing. I only had it flowering once and saved seed. I am now trying for the first time to germinate that seed for about a month, and I think I got one seedling (but 1 out of 20 seeds) and it germinate in the fridge (during 1 month cold stratisfication). But you are lucky; they sell skirret seeds here: http://www.magicgardenseeds.com/SIU01 Its probably cheaper for you to buy it from here, rather than having me sending you.
Walking onion: you can find them in several places; I ordered them in ebay and I am now growing them, but they are still seedlings, so I haven't seen the "walking" activity yet. They germinate easy, just like onions.
Other difficult plants I am havng trouble to find, are mauka, ulluco, mashua and vetiver.
in Portugal, sheltered terraces facing eastwards, high water table, uphill original forest of pines, oaks and chestnuts. 2000m2
in Iceland: converted flat lawn, compacted poor soil, cold, windy, humid climate, cold, short summer. 50m2
this is supposed to be a surprise, but it smells like a tiny ad: