Obenauf's Montana boot grease or Snoseal are both very similar to proofhide and are very suitable for Brooks saddles.

I prefer the Obenauf's. ask for it at the cobbler.

oh, i see you are in japan. maybe a block of beeswax, heated into the leather?

(doesn't REI still have a japanese presence? should be able to get snoseal thru REI..