Depends upon ur budget. Why is she impossible to fit? Is it just the width or is she unusually short backed? What have you tried and why did it not fit?
I use a Thorowgood cob saddle on mine, it has an extra 4th breeching girth strap which helps keep the saddle in place on the flat backed coblet Haffie.
I got it as a temporary thing when breaking her in but 5 years on havent felt the need to move onto anything else!
Def worth a try as they are very saleable due to the changeable gullet if it doesnt work out.
I preferred T'good to Wintec as the tree on the T'good was flatter, less banana shaped and also the wintecs Ive met havent been as good quality (but that might be cos they were older ones)
Ideal H&C (Highland and cob) looked a good fit tho I couldnt afford it so stuck to the Thorowgood! Altho an XXW in the Thorowgood she was only an XW in the Ideal so worth checking/getting fitted.
We have a treeless on our rotund no withers haffie, I forget the brand but it begins with an 'M'. Reasonably lightweight saddle that has molded to her shape. Quite comfy to ride in too.
Before going any further you need to get a qualified saddle fitter to come and make a template that way you know exactly what you need.
My boy had been previously fitted for a M/W then a W but I wasn't happy as saddles kept slipping.
I had a qualified fitter come who spent over half an hour making a full template of his back.
Turns out he is an XXXXW!
She also templated the run from his withers to his loins and form that it was easy to see why most standard trees will dig into his short coupled back! (he is a traditional with a typical peachy bum so perhaps not the same issue with your haffie)
I am having one made by Ray Symmonds to my boys template but fylde saddlery do an off the peg version (called the Ray) based on the wider tree.