You should ideally buy the SR-300 from the Intel online store.
http://click.intel.com/intelrealsense-developer-kit-featuring-sr300.html Intel® RealSense™ Developer Kit (SR300) - Intel® RealSense™ Developer Kits
As far as I know, this is the only official way to purchase a kit. Kits sold by other suppliers may be stock that has been bought by them and then sold onward to their local customers. The only reason to consider buying a kit from a non-Intel company would be if your country was not on the list of countries that Intel will ship the camera to.
Here is the list of countries that Intel will directly ship the SR-300 to:
United States, Argentina, Australia, Austria, Belarus, Belgium, Brazil, Canada, China, Czech Republic, Denmark, Finland, France, Georgia, Germany, Hong Kong, Iceland, India, Indonesia, Ireland, Israel, Italy, Japan, Korea, Latvia, Liechtenstein, Lithuania, Luxembourg, Malaysia, Malta, Mexico, Netherlands, New Zealand, Norway, Philippines, Poland, Portugal, Romania, Russia, Singapore, Slovakia, Slovenia, South Korea, Spain, Sweden, Switzerland, Taiwan, Turkey, UAE, United Kingdom, Vietnam.
