You should have fresh ones soon. Instead of buying them just dry some fresh like you'd dry any other herbs/leaves (spread it on thin layer in a shade, or even just hang the plants, and few days later it's dry and not stinging) and you'll be set for the few winter months while the snow is too much. The young sprigs are way better, so you still may want to dry it in the spring and not wait until later when the plants are older.
I only buy it because of chemical spills/contamination history around where I live and I don't hike too much anymore so whenever I do picking nettles isn't high on my list :)