ebay is your best bet for most of the parts. The wheels crop up occasionally and go for decent money, but still not what I would call excessive. I thinkt he last pair of 10" wides I saw went for £300'ish, which aint bad. I don't think they are extortionate even new from Lexus from what I remember.
I currently have someone who may be taking my spare CD changer off me, but if it turns out they don't need it, I have two spare head units, an amp and a changer. I ditched the factory setup and have my own stuff in mine, something I would thoroughly recommend.
Exhaust parts come up on ebay from time to time too, a brand new back box went for 30 quid or something recently. They are not cheap and there are not many options available after market either.
There werea couple of headlamps on ebay plus numerous vehicles breaking too, so you could try them.