
When looking for iPhone or iPhone 3G replacement parts, there is one thing you should always do, regardless the part you need, and that is to search for broken iPhones for sale. The reason is obvious - no matter how broken one phone could be, there must remain some salvageable parts in it. My suggestion would be to start with your local newspaper, eBay, or some similar local online store.
Now, let's get to the parts you will probably need, once your iPhone starts having more problems than usual, shall we?
1. Battery - officially, there's no internal iPhone battery to be bought, but things look a bit different in reality, as it happens in most similar cases.
Anyway, if you want to play fair and avoid voiding your warranty, you can get a backup battery for iPhone, the good part of the deal being that you can also use it with the iPod Touch. Obviously, it works with both the iPhone and iPhone 3G. Considering that the backup battery available above is priced at $49.95, I consider it a pretty good deal, especially since you won't have to mess with the iPhone's internals...
When it comes to internal iPhone batteries, things are much cheaper, but you need to be a bit of a techie guy (or girl) to install it yourself. Here's a nice 1400mAh iPhone battery I found for you on eBay - OEM NEW Apple iPhone Battery Replacement 1400MAH.
I am sure you can find more than just the above, but I'll stop here with the battery part...
2. Case - my suggestion is to get a protective case, so you won't have to buy some broken iPhone just to replace yours. Here's a nice iPhone aluminum case for the 1st gen iPhone, and since the Web is full of leather, polycarbonate, and even metal cases, all you have to do is find the one that suits your needs.
3. Screen & other internals - best bet would be to simply check for broken iPhones, as I said already, but here's a good iPhone 3G replacement parts source (and 1st gen iPhone, too, of course) - We Love Macs.
As a final suggestion, be sure to check Apple's prices for various replacements, first of all. I say this because sometimes, the time spent to find a cheaper iPhone 3G replacement part may cost you more than grabbing the original in first place and quit wasting time!