Regarding a Catalytic Converter....
 

Hello, here is Julekpl again, and I am in need of some recommendations. I have (and I know, this is quite embarrassing) an AUTOMATIC 1990 miata that needs to have it's Catalytic Converter replaced. I have seen many on the internet ranging from $150-$375, but I don't know which brand is reliable, and not too expensive. Does it also matter that I have an automatic 1.8L engine? Thanks!

Meh, they'll all be about the same when it comes to cheap universal replacements.

Do you have emissions testing where you are? We don't. If not, just unbolt it and take a piece of rebar and a hammer and break the ceramic blocks into small enough pieces that they will come out. Get the little woven steel pieces out also if they are there. Just don't breathe the dust when you are dealing with the ceramic pieces because it contains some potentially harmful heavy metals.

If you really must replace it, Rockauto has one for $99 + shipping. http://www.rockauto.com