I'd check out
Crutchfield. Not the cheapest prices (but competitive), top notch service and tech assistance, necessary adapters, etc. are included in price.
Bad news is I ran your car and model through their system and it looks to me like there is no easy fix outside of a universal changer.
If you can figure out exactly what model of changer you have (google it?) perhaps you can find an exact and cheap (if that word applies to Lexus) replacement for it on ebay. I did this on a Saab changer a while back.
Although I agree if I was doing it today a MP3 connector would have a lot more appeal to me than an external CD changer.