Quick answer: The Zees Belts replacement strap is a real leather strap for adult replica championship belts, sold in 50, 54 and 58 inch lengths for $79.99 with free US shipping. You move your existing plates onto it with a screwdriver. It replaces the stiff synthetic strap found on many mass-market replicas.
Many replica belts have good plates on a stiff synthetic strap that cracks, curls and never sits right on the waist. A real leather strap fixes that for a fraction of the cost of a new belt.