The last time I used HorseSaddleShop they offered to send a shipping label so the return shipping was free. Maybe that was a special at the time I tried a saddle from there. They do have one of the best USED sections for saddles, IMO.HorseSaddleShop has the best return policy that I have seen but it's not a free trial (usually a restocking fee). And you can't actually use the saddle (if you buy a new one) - it needs to stay perfectly new. They do usually waive that in January (so that's now!!!) as a special, but you are responsible for shipping. It can be quite expensive to ship a western saddle.
SmartPak has a handful of saddles in their Test Ride program, but it's limited.
If you can find a tack shop within driving distance, that's your best bet to try it on your horse, sit in it, and see how it feels.
Keep in mind, just like an English saddle, it is important that the Western saddle FITS YOUR HORSE. Yes, I understand he is only 2 and he will grow, but if you put an ill-fitting western saddle on his back that is painful, that's going to be detrimental to your training process.
I'd recommend getting something with the pommel like a western saddle that you can add a bucking strap too. Grabbing the horn doesn't actually keep you stable as it encourages you to pull yourself out of the seat.