I'm 5' 1" and about 105 lbs. I lucked out with an "orphan" saddle I found online three years ago. It's called a Heythorp Heritage, has short flaps, made in England, and is wool flocked. It does have a 16.5 seat, but it fits "small." If you can find another one, buy it. I love mine. I've sent emails to tack stores in England trying to find out who manufactured it, but no one knows anything about it. I have no suggestions about where to find one, unfortunately.
I realize that I'm generalizing here, but as is often the case when I generalize, I don't care. ~ Dave Barry
A 16" will fit in all the French custom saddles. I am 4'11 and about 100 pounds and ride in a 16.5 at the moment and it is a little large but not too bad (currently waiting on the delivery of my new 16"). I'd look for used saddles that have a short flap and are a 16", shouldn't be too too hard to find if you dig around the used saddle sites a little bit!
I have been riding ponies in the Beval Jr. with a medium tree for many years and have really loved it. (I'm 5'1") My other fav is the Butet, with a short flap. Both are available in a wide tree and different seat sizes. I'm looking for a used butet with a short flap, but they are VERY hard to find! The Beval Jr. is easier to find a used one if you like a broken-in version. I bought mine new and I will never sell it!
I'd love to get my hands on a pony butet but they seem next to impossible to come by! A helpful note I picked up on COTH for us short legged riders, apparently, depending on the saddle, it's relatively easy to get the flaps cut down by an inch or so (@Piccoloponies) if done by a qualified saddler. This should help your search for the used butet out some! It's a lot easier to find a 16" in a reg flap than it is in a 0 or 00! Bonus! Special thanks to the COTH community for that excellent tidbit!