I was having a quick look around the iRiverAmerica store and came across their
'close-out' section (sadly, it seems you cannot link directly to the section of the store in question, so to get there you click on 'iRiver Outlet' then 'Closeout Specials').
Within it are a series of LCD remotes for sale at reduced prices. Sadly, this does not include the H300 series remote but it does the
iHp-100 series remote, for $22.
Rather more interestingly, the
SlimX 550 remote, $10 is also available - the same as the iHp-100 remote but sold under a different name.
Obviously, either of these should work for the H300 series too (albeit with the risk of clicking), so it seems like there's some bargains to be had.
There's also LCD remotes for the SlimX 350 and 400 CD players but I have no idea whether either of them would work with any of the iRiver HDD players.