I got her cover from this website:
Equestrian Equipment and Accessories - Saddlery Warehouse
They are based in New Zealand where I live so I didnt order it over the net, I went into the shop itself lol. But in NZ, we measure from wither to base of tail (going along the back) which is the way our covers are fitted. I don't know if they would ship over sea's though if you wanted one, you would have to contact them probably.
That shop is where I get ALL my horse gear from :) they got some pretty cool stuff in there lol