I've had good experiences with shadowofleaves, they sell both cotton and silk ito (the wrap on the handle) and I think they used to sell tsuba as well, I'm not sure if they do anymore. If you're into customizing your katana you may consider even making your own tsuba with a chisel and hammer. Otherwise most sword retailers online carry paul chen tsubas which are good enough (forged iron, never used cast iron tsuba).
There's a site on the net, I can't remember what it's called, they're based in Japan and have an office in Canada, look around on swordforum long enough and you'll find it. They occasionally give away free furniture and sometimes blades with kizu (flaws/scars) to beginner polishers.
you can also buy habaki, koiguchi, etc.. from shadowofleaves, but I think those things tend to be more personalized. You may want to build your own or have someone make them to your specifications if you're going to customize a blade. Chances are a blade you customize won't be used for cutting, it's usually just something pretty to look at and learn from.
Check swordforum.com for sword customization threads, the search function there is great. Remember to register using both your real first and last name.