There’s some differences between boards and kits.
The Intel Edison Breakout board refers to a small computing platform that packs a set of features for developers. Check this link for more information: http://download.intel.com/support/edison/sb/edisonbreakout_hg_331190006.pdf
For “Edison Kit” I guess you’re referring to the Edison Kit for Arduino. This one comes with the Arduino Breakout, which gives the Edison module the ability to interface with Arduino shields or any other board compatible with the Arduino UNO board. In this link you can find some more information: http://download.intel.com/support/edison/sb/edisonarduino_hg_331191007.pdf
Regarding your question about where to buy, you can check this link for a list of distributors near you. https://www-ssl.intel.com/content/www/uk/en/secure/technology-provider/buying/authorized-distributors/overview.html