Buying a saddle online is a big gamble, particuarly when it comes to fit!
You can buy a saddle from a saddlery, have it fitted by a certified saddler, and still not have it fit because it just doesn't suit the shap of the horse no matter how many re-adjustments are made.
I stay away from purchasing online, mainly because I've been bitten alot of times buying saddles and having them repeatedly re packed/fitted. I like to ride in the saddle before I buy as well, to make sure that you feel comfortable in it, they also feel different on each horse.
If you are set on buying online though, I would get a saddler out to take measurements of your horses back, recomend types of saddles that would likely be suitable, and specify what gullet size you need. The standard gullet sizes are narrow, medium, wide and extra wide, with a few brands making 'in betweens'. Then from there usually to get a perfect fit you will need to have it re-packed and adjusted.
If the horse has a high wither, you'll need to find a saddle that is cut to fit a high wither. Not nessacarily a narrow gullet! Also some horses have very narrow shoulders, so the saddle will slip forward onto the shoulders and restrict their movement.
There are so many different issues with fitting saddles, I don't find saddle hunting at all exciting, as it is just so hard to find one that is suited to both myself and my horse.