Being over 300 lbs, I have extremely large legs (20" calves), which makes finding knee-high socks (real socks, not just nylon hose and trouser socks) just about impossible.
I pretty much haven't worn knee high socks since middle school (225 lbs, or thereabouts).
Then, a few years ago, I discovered an online shop, Sock Dreams and fell in love with their socks.
The online store lists sock length and calf and cuff stretch for most socks and the customer sock reviews help too, because reviewers can leave their own calf and thigh measurements and often post pics.
The website has a plus size section, but it doesn't include every sock that might fit plus size legs, so it pays to check out the rest of the site and check out the reviews.
The socks are a bit pricey, IMO, but I'm a bit of a cheapskate, so $10-$20 for a pair of knee-highs seemed kind of crazy at first - which is why it took me a couple years of online window-shopping before I actually placed an order (last winter).
They seem a lot more affordable now that I've worn and fallen in love with them, especially considering that the alternative is no long socks. I didn't realize how much I missed being able to find and wear knee socks (especially during Wisconsin winters).
