best ones I have ever come across are the Tally Ho socks. (part of Horseware now). They are lovely. Padded where they need to be padded with a good arch support and they do knee high ones too as well as ankle and calf length
They're not long (come midway between my knee and ankle and I am short) but I wear Bridgedale Women's Comfort or Women's Endurance Trekker (walking/hiking socks). You can get them in most outdoor clothing shops and Countrywide Stores I think. They've got lovely support at ankles and don't slip down, quite hard wearing too though my wellies eventually rub them through at heels but that's after months. I've not known them to shrink in the wash yet. If you look on their website they have a very wide range so you might find something a bit longer in length to meet your needs.