The purpose of a diffuser is to straighten the air flow exiting the car. This can add downforce, as part of an aero package. (Need a better explanation here.)
Diffusers and rear skirts are grouped together on this page, as some diffusers look like they would be made more efficient with the addition of rear skirts, and some rear skirts look like they would function as rudimentary diffusers.
Please check with the vendor to make sure you can use a combination of these, I did not do any research on that.
You will probably also want to check to make sure your exhaust system will clear the diffuser.
Here is a selection of these parts, please email me if you have additional information not included here.
Thanks to the guys on the RX-7 Forum. A lot of this information came from them; I just collected it here and arranged it by vendor, so it wouldn't be lost.
The first Mazdaspeed one looks like it will attach with the stock rear bumper.
The next series of photos show a different Mazdaspeed style that looks like it will replace the entire rear bumper skin. It appears to have small rear skirts built-in to it.
RE Amemiya makes two different kinds: the "old" one, and the "pro". (Need to get the official name of the "old" one.) First up, some pics of the "old" one:
The following pictures are of the "old" style, on Wael El-Dasher's car:
RE Amemiya makes two different kinds: the "old" one, and the Pro. (Need to get the official name of the "old" one.) Here are some pics of the Pro, many of them from Wael El-Dasher's site:
The RE Amemiya Pro has some optional parts, looks like to increase the efficiency. These are referred to as a bowtech generator on one site.
The first pic is of the diffuser.
This part appears to be a rear bumper that incorporates the diffuser.
This one appears to be different from the other R-Magic ones.
Run bills this as a rear under spoiler:
Similar to the RE Amemiya Pro:
The KnightSports appears to be another one that replaces the entire rear bumper skin (notice the two "vents").
I don't know what brand this is...
The above picture was labeled as rear skirts, but the one below looks like it is the same part, but in this picture it looks more like it was meant to be a diffuser:
The middle photo shows the bumper either cut away or else it is a repalcement skin. However, the rear skirts look like they can be used with the stock rear bumper.
This looks like it is a bumpr cover replacement:
They make rear skirts, but I haven't found a pic yet.