Very little is known about the FT400 on the USA side of the pond. I have only heard mention of them a few times & have only seen one picture that I can recall. The only things that I can suggest is trying your local Honda dealer, eBay, or determine if an FT500 manual will work for the most part. I would think that the FT500 & the FT400 use the same basic engine, with the FT500 having a larger bore, longer stroke, or both.
Good luck
J.