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Most moulding companies have some form of shadow box moulding. Decor has one and most of the other lower priced companies do as well. The best quality I have seen is SouthWinds out of Columbia SC. I can get you a phone number if needed.
Southwinds also has a showcase that can be built up to about 7" deep. Glass on the sides instead of moulding so you can see all around the items. Very nice option to deep shadowboxes.
Fred @ Southwinds: 800-833-3268
Your neighborhood cabinet shop should be able to run a simple shadowbox for you for not much of a charge. Also most larger lumberyards have a mill & can run it for you. If you need a quantity of it, Foster planing mill in Los Angeles can run some more complicated profiles. Nice part about a cabinet shop is they can also give you a simple finish that is of professional quality.
Larson Juhl's 573IG, 573IB, 5731IS is a 4" moulding in gold, black & silver respectively ... makes a beautiful shadowbox. Recently boxed a bottle of wine in the balck one.