Here's my experience buying a replacement battery for my Galaxy S3:
I walk into a Samsung store. I ask to buy a replacement battery. They say they don't stock it and that I should buy it through their web site. So I go onto the Samsung website and did a search and could not find it. I went to a different Samsung store and ask for a replacement battery and said I checked the web site and could not find it. They then told me that it's not on their regular web site. I had to go to the Samsung Parts Canada web site instead. I go on this web site and couldn't find it. I called the toll-free number shown on the web site and they told me they didn't have any in my region (Metro Vancouver).
I eventually just did a Google search and found a web site that sold an extended battery for the GS3. I bought my battery from that web site and I'm happy now.
I never seem to find anything I need at the Samsung store. I once wanted to buy a car mount for my GS3. I wanted just the mount. They only had the whole kit with car charger and all which I didn't need. I ended up getting a generic mount at The Source.