iPhone 3G - replace back cover - where to buy parts

Dear all,
I moved to my new iPhone 4 since the old one (after a replacement of a back cover with a cheap one) after a fall lost the chromed part of volume button and power button.
Now that I do not need my old 3G, I'd like to replace these two parts not being in a hurry.
But...
I'd prefer to go for an "easier" approach.
I'd like to buy another back cover that already includes buttons in order not to have to completely dismantle the phone.
Could you recommend me any on line shop where to buy these parts?
Thansk in advance.

Thanks!!!
I was looking at iFix.
The only thing is that I'd have been happier to find a sort of "back panel kit" with at least buttons and volume controls already mounted to the case