Where can I buy official parts for iPhone 3GS?

It sounds like you did not actually take it in to an "Apple Store". Maybe an Apple reseller? Apple would not tell you you need 'official parts', they would tell you how much they will charge you to repair the glass or replace the phone. There is no other option at an actual Apple Store. Find a real Apple Store and make an appointment at the Genius bar.

An Apple store asked YOU to provide them with parts? That doesn't sound right...

If you were indeed told that you need to provide your own parts, there's no way you were at a genuine Apple Store, one run by Apple themselves. Any employee of a real Apple Store will know that Apple does not sell parts to end users. And any authorized Apple service center would know the same thing and would know the procedure for getting an iPhone serviced, which definitely does not include the customer providing parts (iPhones aren't repaired in the field; they're exchanged).

The only scenario I can think of is that they were telling you that they can't do anything with your iPhone since it has parts for an unauthorized source, and that only if those parts were replaced with the original factory parts could they help. If that's the case, then unless you can replace the digitizer and any other parts that were not Apple-original, you're probably not going to be able to get your iPhone serviced by Apple and will have to look to an independent repair service.

Regards.

Message was edited by: Dave Sawyer

There is no place where you can be guaranteed to get only official Apple parts, since again Apple does not sell parts. They don't even provide them to authorized Apple service centers, since such centers do not repair iPhones, they just exchange them and send the failed/broken units back to Apple.

If you want to order parts, there are a number of places you can do so - just search the web for "iPhone parts". But whether they'll be "genuine" or not as far as Apple is concerned there's no way of being certain.

Regards.