With a 15 inch saddle you'd be limited to children's saddles which tend to be inferior in materials and construction or a pricey custom. If you can move up to a 16 inch, there are "off the shelf" saddles out there. There are more choices if you look at dressage saddles instead of A/P or huntseat saddles. Dressage saddles also tend to be smaller so that's might make it a decent fit seat wise. Saddle makers all have their own way of measuring and marking trees but look for a 32 cm or larger, MW (medium wide) W (wide) XW.... or maybe 3,4 or 5 as a place to start. Stubbens have a reputation as wider saddles than other makers (a 32 Stubben will be wider than a 32 X). My 32 Stubben Sigfried fit my full Percherons until they were 4-5. Duetts have a decent line of 34 and larger saddles. One of my boarders rides her full draft in a 34 Duett (don't know the model) dressage saddle. I have a 35 Tony Slatter that 2 fit in but need to find something wider for one mare.
Stay away from anything on eBay that claims to be a draft saddle. You get what you pay for. Just like shopping for a light horse, spend the $$ and get a decent quality saddle or you'll be disappointed. I found my Tony Slatter for $400 at a Hardware store !! So you never know where you'll find one.