They're called waraji. You might have luck searching ebay or google under that term.
If not, go to the bleach cosplay thread, then search it for "waraji". There will be a lot of hits, but there are several posts with links to tutorials. Or search the props and accessories forum, I think, for someone posted their waraji tutorial a few months ago.
[font="Franklin Gothic Medium"]~Anime North 2014 Masquerade Co-Director [Masquerade AT animenorth.com]
~ Anime North 2014 Culture Track Organizer
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