I mentioned stiffeners, as did others. But you can also find what I call double thickness belts (basically two pieces of leather glued together) that are also heavy enough for a good gun belt.
Oh, and with regard to leather, it comes in different thickness. Try to get the thickest leather you can. It will not only work better, but will last longer. Truth is leather belts do stretch and wear out over time. But start with one that is well made from good materials and it will last a very long time if you take care of it.
We could do a whole thread on leather care (probably been done already). But like a firearm, leather needs care. But no manmade substance has the feel, smell, and even sometimes the sound (for ccw creaking leather might not be good, but for some reason the creak of leather just seems to go with the out of doors and especially horses) of good leather made into top rate products.