Try buying a new saddle

Ok, so not everyone goes through this problem, and you must be very glad you dont.
Trying to get a new saddle that will fit my ever growing rear end. My 30 year old saddle still looks new, and fits every horse like a glove, but my rear end seems to be falling out over the cantle, so i decided it was time to search for a bigger 16 or 17 in seat.
I found one on craigslist, happens to be my daughters old equestrian coach, and so i go over and get the saddle and try it on my little coming 3 year old filly.
Well, she must have had that saddle fitted to some kind of round barrel, because the sides flap way out on my horse, whom is perfectly conformed. You cant just use anysaddle. I do not want that to be the reason why Perla (the horse)decides to buck. So my search continues to find that saddle, that is light, brown, and designed to fit any horse!