You need to try a dance supply company - maybe you can find something on amazon? these are pretty popular with ballerinas, but they can be quite expensive.

PP again!
Here you go:
http://store.americanapparel.net/long-leg-warmer_rsalwl

they also have 3 pack for 39 I think online. They look scrunched down a lot, I would be willing to bet they are much longer than the way they are styled based on those crazy socks I bought.

I bought some "over the knee" socks from American Apparel a few year s back (maybe 4?) for a costume party for a friend's birthday. They are really, really thick knit which is nice and came in dark grey (not just their screaming neons!) but they seriously could come up to almost my crotch. No kidding. I have a 33" inseam and am about 5'9"

So they are WAY over the knee. Which was really weird, I had to fold them down over a foot! But all this is to say that they may have legwarmers and if they don't that you could buy these socks and cut/seam them to whatever length you want. Would be really warm over tights and boots and then stretch off.

I've never seen over-the-knee legwarmers.

Could you buy two pairs the same then sew two together to give one extra-long one?

Where can I get some longer-length leg warmers, that extend past the knee? That are comfortable, simple (not bright/flashy) not super pricey, and durable/machine washable?

[Yes, I'm aware that some people hate them. I don't particularly care about their opinions when I'm walking 3 miles in sub-freezing temps!]