Is there any difference in the rotor housing from s4 to s5, and from na to turbo?

Also, does anyone know a good place to buy used housings? New ones from mazdatrix are a bit pricey, and I don't quite trust used ones from ebay...

 

I believe the spark plug locations are slightly different from s4 to s5 for the NA. they can be interchanged though.

I think the turbos are the same.
Of course the exhuast ports are different from NA to turbo. NA has the diffuser. using a turbo housing on an NA engine would be a good thing.

check with an engine builder to be sure, they might have decent used housings too.

 

trickfc is correct. However, if you have an S5 you would want to put the same type housing back. It has to do with being high compression. You don't want to have one of each (1S4 & 1S5). Turbo housing will work on an N/A if you plug them.
There is a guy here (mr Mazda) in Dallas that could help you with housing. Builders here usually don't sell them as they want them for their rebuilts. Ebay sometimes will have some.

 

The rotors are different and are the reason for the compression difference - not the housings.