I am a larger lady myself, so I understand your dilema . One problem you may run in to is that the larger seat, the longer the saddle. Most saddles 18" or larger are going to be too long for most horses' backs. My 17" Circle Y is almost too long for my pretty average sized 15.2h Appy/QH mare. Drafts and draft crosses often have shorter backs than lighter horses.
Measure on your horse between the back edge of the shoulder blade to the middle of the flank (where the hair grows up), and add 1". That is the longest saddle that will fit on your horse.
When looking for a saddle, look for one that is made in North America on a quality NON-FLEX tree, preferably Ralide or Hide covered hardwood. The tree is the most important thing on your saddle, especially for those of us over 200 lbs . You want a good sturdy tree, NO fiberglass or flex materials, and no "soft woods" like pine.
If you cannot find a quality western saddle in your budget, you might consider an Aussie saddle. Down Under Web has a nice selection and they might be able to build one for you. Aussie saddles are shorter front to back, and you can still have the horn and security of a wetsern saddle.
Saddle pads are also very important for heavy riders. I like to use Skito saddle pads. They're very shock absorbing, circulate air, and help keep the horse's back comfy. They have a "firm foam" that works very well for heavy riders. They have pads for western or aussie saddles (English too).
Please feel free to PM me privately if you'd like .