The i747 is the NA variant of the S III that has a 1.5GHz dual-core Snapdragon 400 processor with 2 GB of RAM and LTE but no FM radio.
The i9300 is the international model with 1GB of RAM and no LTE but has a quad-core Exynos processor and an FM radio instead.
You'll basically have to find the i9300 online or find someone selling it in your local cell phone shop, mall, etc.
Samsung is skipping 4.2 and going straight to 4.3 for the S III and Note II, but it's not out yet (unless you root and install your own rom).
I haven't seen Photosphere running on any other device or rom besides stock Android on a Nexus 4. LG has their own version of the program called "VR panorama" on my G2 and I just tried it but it doesn't seem to work as well as Photosphere. Maybe I just have shaky hands today...