Well, lets start off by saying good that you did your research on the item. And if my answer is correct, please let me know. When you open your iPhone 3g(www.ifixit.com will show you how) and pull off you your entice display enclosure, you will see the Cable dock flex cable. Disconnect this from your logic board and make sure to unscrew the necessary screw. What you have is a combination of three part that you have just taken out because they are all connected. First was the dock connector flex cable with the left speaker attached. Second is the sound buzzer/bottom speaker en-closer. It's all black and not that big. On the bottom of that, which is the third part (as well as the part you are looking for)if your antenna flew cable. This is the part you want to replace. Now if you think if you can just buy this item and replace it, your right but wrong. There is metal prong that is connect from the antenna flex cable to the buzzer. The buzzer is also connect to the dock flex cable. In simple terms, you want to but the piece already assembled so you will not have to deal with the hassle like i did (talking from personal experience). Now where can you buy this, thus far, ifixit.com has not been able to accommodate for this item, but eBay has. Since you are in India, i think eBay would work best for you. I also think you should purchase from somewhere in Thailand or Tokyo. there items are usual cheaper and also usually have free shipping. Hope this has been of help.

see the teardown guide for iPhone 3G, step 18 for the location of the antenna flex, or the repair guide step 17 - it's on the back of the dock connector assembly

and for reference, look at this other picture of all the 3G parts - you can see a small picture of the antenna there