Recently I bought a Tsugaru Shamisen online and when I received it the Itomaki was cracked and wouldn’t stay tight. Due to the fact that I bought it from a dealer in Japan who sells antique and used shamisen it was impractical to try and return it because I would just end up throwing away more money. My question is this; where do I get replacement Itomaki, and what is the best kind to get (material&pattern)?
You should try to ask ChordsCanada.com. Even if they don’t have itomaki in their catalogue, they may have some spare ones in stock.
I’ve seen recently a few ebony itomaki sets on Yahoo Japan Auction but found nothing today; they have been sold. Strangely enough, they are rarer than the ivory ones O_o. If you’re looking for the super special awesome and expensive acrylic ones they can be found very easily on YJA, and on Rakuten as well.
Thats a Kyle question if ive ever heard one.
However, before I was good friends with Kyle, I incidentally ended up with a broken itomaki. I ended up meeting up with a violin maker, and he glued it back together for me. It lasted probably 6 months after that before it broke again. Lucky for me, Masahiro left one of his extras, and he let me have it. It fit, so I lucked out.
Also, if You have a way to get a hold of an ebony blank, you could possibly find a wood worker or luthier who can help you. Its always good to check around your local instrument shops, because chances are, they know someone.
Actually, yesterday I was going to reply with, “That’s a question for Grant, the itomaki aficionado .” 
Anywho, Grant answered it bang on!