I bought my RX8 used here in Huntsville and after a few months I realized the wheels had locks on them. I went to the dealer and asked for the key and they told me nicely that the car comes as is, nothing more, nothing less.

I suppose my question is where can I get a replacement key? (My car did not come with a "registration card" that I've heard about, however, the dealership told me that my wheel lock number is 166)

Can anyone help me out?

 

Any Mazda dealer can get it.
Mine took their key collection (24 keys in the case) and went through each one until they found it.
From there they knew which key to order.
I actually bought a new set (4 lugs and key) for $11 on ebay and asked to borrow the dealer's key and swapped it in their parking lot.