amayama.com, based in Japan, sell genuine parts at a lower cost than our main dealers. Apparently they work out cheaper even after allowing for shipping, duty and VAT, although I have not used them myself.
Another option is rockauto.com in the US, who also sell aftermarket parts. The same caveat about import costs also apply though.
Assuming the rear suspension is not dissimilar to the saloons it might be possible to source bushes only for some components, whereas Lexus only sell complete assemblies. It probably depends on whether or not you want to stick with Lexus sourced parts.
Finding a reliable alternative source of parts in the UK is difficult, unlike BMW and Mercedes-Benz, for example.