Amayama price is £253.26 but then add on shipping plus UK import taxes and you're not far off UK Lexus prices, problem is the car was only made for a year so there will never be an aftermarket alternative.
If you can find a good welding place it may be worth having it welded as the pipe is very similar to the Y pipe on the LS400 a known weak spot.
Just goes to show how cuts have been made in quality, my LS400 is now 22 years old and on the same exhaust system from new.
Part numer for the back boxes is 17430-31540, the gasket (17451-31010) comes up at around £35.00 but seems to be incorrectly priced, the rubber mountings should be reuseable.