OK, well, this might seem kinda silly, but there's this doll making site that shares some patterns for anime inspired dolls & clothes. It could at least give you some ideas of what shape the fabric needs to be cut to make the items you want (sized up for a real person of course).
http://dollmaker.nunodoll.com/
Scroll down to the Kenshin & Inuyasha. There's a Sai with good photos, but it doesn't have directions.
You could also try looking at Folkwear patterns to see if they have something you can use.
http://www.folkwear.com/