Replacement key options
 

The key on the left only works the doors but won't start the car (both fobs work). I assume the key on the right has some kind of transponder in it.

What's the process for getting a replacement ignition key?

99MY Impreza Turbo.

Attachment 36761

Subaru dealer?

Is that the only option? I'm not against it, but my expectation is it won't be cheap if a dealer is involved.

Ask them first and see what they say.

If it is too expensive there are people out there who can copy the key and code it, but make sure you do your research, what's to stop them making a copy themselves and taking your beautiful car?

You can get a replacement key from Timpsons on most high streets. They sell the key blank with transponder inside and copy the code from your working key.

What about this:

buying a key from any Impreza MY99 and let a dealer reprogram the ecu so it will recognize the new key? Possible?

The keys on this year are programmed to transponder receiving unit, not the ECU. Plus Subaru dealers are pretty thin on the ground, round my way anyway.
Timpsons couldn't copy these a few years ago, maybe they can now.