There are two squirrels in Britain, the Grey Sqirrel, and the Red Squirrel, which is indigenous. Most old patterns refer to the Red Squirrel! One may substitute practically any fine red brown fur for this. Or red brown Mohair. Fox ear is good, and the reddish brown fur on a hare´s mask is also very close, although there is not much of it. This of course assumes that red fur is meant. Red squirrels have blue/grey underfur! The tails are used for many streamers, and the body fur mainly for dubbing.
Grey squirrel tail is a "hard" hair, which means it is smooth, slippery, and incompressible, so various tricks are used when tying it in to make sure it stays put!
Here are from left, to right, a Grey Squirrel tail, an Asian Squirrel tail, and a Red Squirrel tail. The Asian Squirrel tails are useful for some things, but there is a lot of black hair on them, which of course limits their use. They are often offered as "red" squirrel tails, but of course they differ considerably from the original, as shown below.
Other varieties of squirrel tails and pelts are also offered in other countries, but I have little or no experience of these, and no pictures!
(http://img580.imageshack.us/img580/9408/stn1.jpg) (http://imageshack.us/photo/my-images/580/stn1.jpg/)
The grey squirrel tails are also offered in a large range of dyed colours, sometimes also previously bleached. Here is a range of unbleached dyed tails. The quality can vary massively. Long haired tails are usually sought after as one may of course dress larger streamers with them; One may of course also use the underfur as dubbing, and the guard hair for wings or hackles on various dry flies etc. The guard hair and underfur from the body skins may also be used for dubbing mixtures, or also as wings etc on dry flies.
(http://img856.imageshack.us/img856/9506/cst1.jpg) (http://imageshack.us/photo/my-images/856/cst1.jpg/)