This can be quite tricky, as no two manufacturers' of stockings had the same measurements! As a general rule of thumb your vintage stocking size can be worked out from your height and shoe size, but this is not infalible! Long length stockings are always popular, the modern woman on average has longer legs than those of the 40s and 50s.
Ebay can be a good source but finding those longer lengths is difficult.
So, for your shopping, as well as Kay Marel (see previous post), try:
A recent find on Ebay: