I would like to begin collecting katanas, anyone knows where i can buy?

If you are thinking of collecting bonafide, catelogged swords that are registered then you need to start saving your money. Swords like that have been declared national treasures by the country of Japan and you can't buy one there and take it out of the country without paying for an export license and fee. Swords like that start at what you would basically pay for a decent car and go up from there to even into the millions depending on their lineage, and how ornate they are and what condition they are in. Otherwise you can buy cheap, stamped out reproductions for a few hundred dollars apiece. They also make foraged swords and you might check out Bugei Trading Company. There cheapest foraged sword starts out at about a grand or so and their web site is below.

http://www.bugei.com/

I believe Iaido is usually the ones doing the cutting tests. You can look up iaido equipment and you should be able to find the swords used. I believe there are also non-cutting iaido katana used for form practice and such that are lighter and make with non-steel alloy, so check if it's for cutting and if it is made of steel.

I think collection worthy katana will be crafted by master japanese swordsmith or a historical piece, but that might run became a something of an investment.

If you don't care for the historical or martial side of the katana so much then there are companies out there like cold steel that are manufacturing katana that can with stand the cutting test and then some. But from what I understand the weight and balance is off compared to what a traditional katana would be like.