We have a multitude of diffusers and grilles available through the brand Vents-US, a leading brand in the ventilation industry. They have a number of different styles available to choose from. Their MV PFs series of diffuser (the ones literally named "Plastic Round Diffuser") would be your best choice for a 4-way diffuser. Sizes up to 14 3/5" in diameter (see MV315PFs) are available for purchase.
If this isn't the type of diffuser you are looking for, Daikin has concentric diffusers available as well that attach onto their package units. These are not the same as the ones you would find in a ceiling. The concentric diffuser has two pipes in it: one for the supply duct and one for the return duct. If this is something you're looking for, please give us a call and we can create a special order for you.
If you have any further questions, please call us and we will be happy to assist you.