l just bought a 99 Lexus ES300 that only has the valet key, and l was told by the dealer that l could get a new master key and remote on ebay, which would then be easily programmable.

call lexus and ask em

If your valet key is programmed as a master key then it should not be a problem to program another key. If however it is programed as a sub key, as l suspect it maybe, you wont be able to program another master key. l suggest you go back to whoever you bought the car from and see if they can find you at least one of the original master keys. Otherwise you will probably have to replace the lock set + engine ECU which is very expensive