I'm playing a ninja-themed character, but I haven't been able to find a decent (unenchanted) katana, or any katana for that matter (other than the enchanted ones sold in the imperial market). They're listed in the wiki, but I have no idea when they should drop. I'm already seeing elven long swords, and they're more powerful. I would think I would have seen at least one katana.

Also, I'm going without armor, but I can't find any decent clothing for him. Everything is either robes, potato sacks or ren fair outfits. The closest I could find was a pair of black pants and a dark shirt, but (despite both being black) they don't look right together. The shirt is fine, but the black pants look like denim bell bottom capris or something. Can anyone suggest anything better? Thanks.

I'm pretty sure you can only get Katanas by doing the Main Quest.

As for pants, have you tried Blacksmith Pants or Laced Leather Pants?

Laced leather pants look too rockstarish for a ninja. And the blacksmith pants look like ordinary black denim jeans, which are never cool...not even in a mythical video game fantasy world.

If I'm not wrong, only the Blades have Katanas. So start the main quest.

Is it possible to sneak out one of the Blades katanas from the tutorial dungeon? I was thinking I could stash one in a chest or on one of the assassins bodies while I have that character creation dialogue with the Blade, then getting it back after.

You can drop it before Baurus asks for it back, iirc.