Get a copy made at a key shop, $5 and you can program it yourself in 30 seconds. See of you can find a shop that will cut a key off that tag with numbers that came with your keys, it'll be a better cut. If not use one that's as new as possible.
1. Insert original key (one that's already programmed)
2. Turn to "ON".
3. Turn to "OFF" and remove key.
4. Insert new key.
5. Turn to "ON".
6. Turn to "OFF" and remove key.
7. Insert original key.
8. Turn to "ON".
9. Turn to "OFF" and remove key.
10. Insert new key.
11. Start car.