Dear Mark,
I have had the exact same difficulty getting hold of a Mesa Red Touch-Up!!! However, I think I have the answer to the problem. Lexus Parts Sale standard routine when searching for a Touch-Up paint is to add a '0'(Zero) to the end of the paint code. This long standing routine normally works. However, with Mesa Red and other new colours the routine has changed and they must now add a 1(one) instead of a Zero.
Therefore, the touch-up does exist but its part number is causing difficulty for Lexus operators to locate on the database, so they tell you it doesn't exist - but it does!!! I hope this helps as Mesa Red is the best colour so we have to keep them looking cool