From the mk3/4
Ibiza forums if you have the transponder in the non-Remote keys all you need is to buy 2 flip keys off ebay.
The transponder is what turns off the immobiliser and is a little glass tube bonded into the plastic of the key - the risk is that you can break the transponded and the you
WILL need to get a new key from SEAT.
The key that you buy from ebay has to match the frequency of the RC key you originally have
(now here's the problem?? Not sure how you would tell what you need if you've never had one??) But surely all Leons came with remote central locking?? You can then code the key to you car by certains turns and clicks and or vagcomm, but I will need to look out the old thread to see for sure?