One of these days, I'm going to find a saddle that fits my girl - one of these might work, she's high withered, broad backed, and downhill to boot. Sadly, the seat sizes for these are just too small for my fat backside!
You should try the saddle I'm riding in now, her back sounds a lot like my Milly's and it might fit her perfect :)
I learned a lot about saddle fit from the local tack shop, and they showed me the exact style to look for. You should see what they say and look for something similar like I'm doing.