there are one that works just as fine, I've been using it and it's just as good as Mazda's (which is really just NGK)

NGK is one example, it works just as fine and it's cheaper

there are other brands too, which is really just rebrand of NGK sensors with a cheaper price tag, and yes it works the same too.

just gotta explore...

 

Note that I said "buy oem", not "buy from dealer".

You agreed with my statement by saying "buy NGK". It's still oem, even if not bought from a dealer. It's always on the owner to research where best to buy from.