Instructions
Check your local martial arts or military surplus store (flea market stalls can also sometimes stock the rare quality katana). Examine the quality of the blade and handle of each sword. Determine whether the "tsuka" or handle is made with plastic or resin, and observe how many metal pins are used to fasten the handle to the blade tang. Study the temper line of the sword blade: avoid any sword that seems mass-produced, uses plastic for construction or has "440 stainless steel" inscribed near the hilt (440-grade steel, while fine for kitchen knives, is unsuitable for swords due to weak grain boundaries caused by chromium).
Search online auction houses such as eBay for the swords offered by various independent weapons dealers. Avoid merchants who refuse to offer refunds for merchandise defects, or who do not include very detailed photos of the blade, scabbard and handle on request. This is good for finding deals, as well as rare finds. but it is often difficult to test the quality of an online product.
Search directly the websites or catalogs of the upper-end katana manufacturers and swordsmiths like Paul Chen, Bugei or Cold Steel. All of these are made with RC60 strength and above, which means it's a spring steel that utilized few porous elements during forging to reduce brittleness.
Seek a custom weapons-crafter for your sword. Bugei offers traditional blades as well as custom orders of various handle, blade and scabbard lengths and thickness: down to the very shape of the "hama" or temper line that runs along the blade.