1. Make sure gun in unloaded and hammer is fully forward (if not, the hammer spring will have extra tension on the backstrap when you try to remove it)
2. Push pin out at the bottom rear of the magwell
3. Pull the bottom of the backstrap out/backwards
4. The grip panels then pop off the sides
Sorry, no idea about where or what is available for replacements.
(Editted to add step 1)