Individual disks or membrane diffusers with combined base or air station are usually assembled / produced by places that sell complete aeration packages. These places produce their own favorite style of combination diffuser by assembling parts for profit. A few pond management companies will sell the individual disks. Web surfing can be informative for this topic.

Remember that when you lift or mount the diffuser off the bottom DURING SUMMER STRATIFICATION my testing shows that the pond circulation moves down to the depth of the disk/diffuser and very little if any depth below the disk. Thus if the diffuser is 2 ft off the deepest bottom,,, 2 ft of the pond bottom zone usually does not get mixed. Try it and do some measurements for yourself to see how your pond circulation behaves. IMO this information is not absolutely correct during winter conditions. This is because water density and circulation characteristics change significantly between summer 80F+ and 39F.