iTunes isn't actually a distributor. (It's a store.)
There are actual companies that distribute to iTunes, for a possibly outdated list see
http://www.amazon.co.jp/gp/aw/help/id=200850070
I've crawled iTunes to identify 3 of those:
http://www.rightsscale.co.jp/ - 45802042***** barcodes
http://www.elicense.co.jp/ - 45801221***** barcodes in 2012, I do not know what barcode range they moved to in 2013.
http://www.crypton.co.jp/ - 45118209***** barcodes, mostly assigned to
http://karent.jp/ but not exclusively so