If you don't have any luck finding what you are looking for, email me, I can easily tell you how to make your shadowbox moulding from supplies that are locally available at your local home repair store. Our big ones here in the states include Home Depot and Lowe's but any of that type of store will have clear wood in "one by" lumber. You may have to size your boards but, with a little pre-planning and an idea of the dimensioning of lumber, you can cut and finish your shadowbox moulding with nothing more than a good quality table saw.
And, chances are if you know how to shop with grain selection in mind, you will have a frame that will stay straighter than much of the imported mouldings available on the framing market today.