http://www.jr-odekake.net/en/icoca-haruka/

It is not like the Japan Rail Pass or other JR tourist passes which you have to buy outside Japan. You can only buy it inside Japan -- specifically, at the JR ticket office at the KIX train station. I think they should let you use your credit card, though you should have some yen with you anyway.

The reason that it's so much cheaper than buying a regular ticket alone: it is limited to foreign tourists. Japanese nationals and foreign residents of Japan can't buy it; they check your passport and return ticket at the time of purchase. This is actually more restrictive than the equivalent NEX+Suica package in Tokyo, which foreign residents can buy.

Now, jury is out as to whether the ICOCA+Haruka is better than the 3-day Kansai pass. It depends on how much you plan to travel around while you are in the region. One thing to note, though, is that the 3-day pass only works on JR lines, while the ICOCA card works on pretty much every mode of transport. ICOCA is more useful for getting around Kyoto as you can ride the city subway, the local buses, and the local private railways with it; JR does not really reach inside the city at all and is mainly useful for getting in and out.