I'm sure some have seen this, but i know they were hard to get a while back when Franks was moving.
https://hvccycle.net/fork-tubes-yamaha-rd-250-350-1973-1975-351-23124-50-00/?mc_cid=ee6f369573&mc_eid=fa6d6d5c4e
Niche Cycle sells fork tubes for RD's: http://www.nichecycle.com/ncs/categories/suspension/fork-tubes/ncs4210-yamaha-rd400-34mm-fork-tube-chrome-by-niche-cycle-supply.html
And FFTP:
https://www.fastfromthepast.com/fork-tubes/fork-tubes-chrome/yamaha?p=1 (https://www.fastfromthepast.com/fork-tubes/fork-tubes-chrome/yamaha?p=1)