There are no substitutes for the Kato decoders. NGDCC doesn't make anything compatible. Unless you want to hack-and-slash, your best bet is to wait for them to come back in stock (they will, eventually, I promise! Their manufacture seems to be on a 6-month rotation).
That said, other members have been able to order from NGDCC by emailing Shuhei Nagasue directly (email address is at the NGDCC website
http://www.snjpn.com/index.htm ).