You're not going to like this.
You can't get another master key if you currently do not have one. The way the Lexus system is designed you need ONE master key to program other master keys. Since you only have a valet key you need to replace the engine ECU to program new master keys, a $1500 job.
Now, some say Lexus will pay to do this ONE time in a vehicles life, may want to look into that, otherwise I dont know what to tell you.