Have no idea where to get just the grill, However you could get some cheap in-walls from Monoprice and use the grills for your intended purpose. Obviously that's more than just what i would imagine a grill would cost but it will still get the job done. HTH
another option is to use AC duct covers... looks like they are supposed to be there and allow you easy access when you need. Speaker grills may raise the question, why do you have in ceiling speaker way over there in the corner... and that corner and back over there.
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