The description for katanas warns you that "magical Katanas are VERY RARE," meaning there's probably more than one. I'm sure they threw in one or two good ones but you'd probably have to look really hard for them.
If you want a Charname with a +1 katana from the beginning, but who/whom is still level 1, you'll have to do a little bit of runaround I think.
Here's what I'm thinking. Play black pits until +1 gear becomes available (after the fifth advancement match). Get the +1 katana (and any other gear you like). Save. Export the character (call the file Gearey or something). Start a multiplayer game. For the first character, make the Charname that you want to play the game in Singleplayer mode with. For the second, import Gearey. Start the game. Give the gear to Charname. Save. Export Charname. Start a single player game and import your Charname with the gear.
Note, however, that I'm pretty sure +1 Katana is as good as it gets in BG1/BGEE. That's it until BG2. They were not kidding when they said rare.
People have asked overhaul to make them a little less rare (perhaps giving Ogre Magi katanas, since apparently ogre magi are easy to find and carry katanas according to the monster manual). . . . Overhaul lol'd.
Never seen a crowd of people waving "God hates burglars" signs.