Normally I would say from your local custom bike maker, but yours is probably in Japan. You may find it easier to buy a Ti or Carbon one.
henryjames.com has an index of some US custom builders. There are some good custom MTB workshops in the UK, esp Dave Yates at mrsteel.
If you are just riding, not doing any manic off-roading then a 26" touring fork may do the trick. These are lighter and springier than conventional MTB steels forks, but designed for heavily laden bikes and unsurfaced trails. Bruce Gorden in the US makes good ones.

Id be interested to see how a custom steel fork fared against Ti or carbon.