Here are all the outlets I know of. Please let me know if you spot any others.
Most of these offer paperback and e-book options. The paperback is Print-on-Demand so may take a little longer to arrive. In case of urgency please contact me direct as I'll usually have copies on hand. Shipping to the UK is easy; in other areas you're probably better off looking for an alternative.
For Apple Books readers (like me), go here.
The Sundering Wall is here.