I was in the same boat as you when I picked up my 05 LGT.
I went on Ebay, picked up some immobilizer keys, took it down to the dealer and had them both cut and program the new keys.
It cost me $7/key for them to cut (I had them do it so they took responsibility for the cut should there be an error)...and they charged me $21 to program all 3 keys (all keys need to be present since they have to re-program all at once).
One thing to remember is that only 3 keys can be coded at a time...if you get a fourth, it just kicks out the other...so there is no sense to have more than 3 keys.
Here is where I bought mine...worked like a charm and 1/5 the cost of buying a key at Subaru.
The seller was: mykeyless-1