Where can I buy another 4 button programmable keyless enrtry remotes
Parts department at your dealership, many online stores and or ebay.
You're best bet is to go to a dealership so they can get everything programmed etc.
Get it online and then take it to the dealer to have the key portion cut. If you just want to program the keyless entry piece, I believe you can do that with instructions from an online search. But in order to start the car with the cut key you will need to have it programed at the dealership.
Looking for a Used MAZDA3 in your area?
CarGurus has 27,029 nationwide MAZDA3 listings starting at $1,999.
Search Mazda MAZDA3 Questions
Mazda MAZDA3 Experts