I made a 3rd key for my 2007 mazda 5...
I bought a transmitter fob off ebay (~$30). I programmed it using instructions in the PDF file. You can program up to 3 fobs for your car.
-Note that although the PDF specifies it is only for 2006 vehicles below a certain VIN, it worked fine on my 2007.
I bought the RFID key blank from the dealer (~$10?) and had it cut at the local locksmith ($5). You or locksmith will need to tap in the small roll pin to attach key to the fob if it's not already attached. I then programmed the key using instructions in the GIF file. You can program up to 8 blanks this way.