Hi there i own a Mazda 3 TS 2006 1.6 Petrol.

My EML has come on (Cat Below Threshold) I am told that i need a new Cat. Where is the best place to get one from and what do i need to buy?

 

That entirely depends on what sort of emissions requirements you have. If you don't have any emissions requirements, just go with the bare minimum. Otherwise, OE is a good way to go, which you can pick up from somewhere like here.