You have a couple of choices;
-see if your local Lexus will flash your ECU for free. Sewell Lexus used to, don't know if they still do.
-some locksmiths will flash the ECU if they have the tool, but it's not going to be cheap.
-go to car-part.com and look for an engine ECU/master key combination that will match your car.

I have done the flash route through Sewell, via mail, worked perfectly.

 

Do not know where you are at on this with 2 threads, so anyway:

If you have a master, you can program other masters. Also, don't confuse the "electronics" with a transponder chip. The chip is fairly small, and sits in the electronics assembly on 3-button remotes, but is by itself in the hard-cased keys (I know, duh, right?).

Anyway, it's the transponder that replies to the immobilizer, and it's the transponder number that gets programmed into the ECU.

As mentioned on the other thread, if no master, than you'll need to get the ECU flashed and then look up the pedal-dance instructions for adding keys to the ECU.

 

Unfortunately, I have no master key, so my only choice is the reflash...

 

Sucks. There are 2 Sewell's, on in Ft. Worth and one in Dallas. The one that assisted me was in Dallas. Good luck with your efforts.