Hmmm well if you want a DEEP seat the new martha joseys have a 5" cantle and taller swell, but they have the SUPER forward stirrups that I, personally, can not stand. They run somewhere around $2000 new. You'd be hard pressed to find one used. Double J Pozzi's are deep too. You could find a used one in the $1000-$1500 range, but then again finding a larger seat is more difficult (trust me, I know!)
For my saddle, I bought a Marlene Mcrae Special Effx. It's one of the new ones made by cactus, and I had it customized. With my customizations, it was $1800 and I love it. It has a fairly deep seat, but not super deep. The stirrups hang underneath you, and the flex tree in it is one of the best around. It has fit every horse I know except one with only an adjustment of pads. Right now I'm using it on a gigantic HIGH withered TB and before I was using it on a medium withered wider qh. Mine has roughout jockeys and fenders and a suede seat, so even though it isn't super super deep, I find it fairly secure. The qh I mentioned earlier had a dumb moment and stopped dead, spun around, took off, then reared up and I literally did not move at all or lose my balance at all in that saddle.
OOH! Another one came to mind just now. I don't know what you're willing to spend, but Deb Sibley make some REALLY cool saddles. They have a flex tree that is supposed to fit most horses. You can custom order them with all kinds of features including a 5" seat and owners say they are very secure. They also have fully adjustable fenders so you can adjust how forward they are as well as length. These saddles start around $1600.