To re-iterate some of the points made here...
1. You can buy a Lexus Platinum Warranty from a Lexus dealer other than the one where you purchased your vehicle.
2. The prices are very negotiable. Do not take the first offer any more than you would take the first offer for the price of a car.
3. There are always better priced non-Lexus warranties available. Some work well and others may be problematical to collect on. (Note: I did learn an expensive lesson when a BMW vehicle with a non-BMW warranty was not supported as the warranty company went bankrupt and I was told to get in line for $ when facing a $1,200 repair. This same company is still offering 3rd party warranties under the same name, claiming the underwriter is who actually went bankrupt.)
4. It is better to get your extended warranty as early in the life of the vehicle as possible. The dealer showed me his pricing schedule which gets more expensive as you get closer to the 48 month factory period. Again, not sure how much trust you can put in those numbers. One thing that is worth thinking about is that the Platinum warranty from Lexus actually extends the umbrella of coverage to things not included in the factory warranty.
Remember on all of this YMMV (your mileage may vary) so do your research and make the decision that works for you!.
Steve