Can't help with the boxes I'm afraid, and you probably couldn't afford to import them from the UK even if I could find them!
But a couple of other (hopefully) useful tips.
1. It's worth storing the tapes "Tail-Out". This prevents print through, which can result in a pre-cho as the magnetism seeps through the wind of the tape..Comprenez??!!
If this has already happened, there's not much that can be done sadly!
2. If you have access to a variable speed machine, and have to spool through the tapes. Do it at as slow a speed as is possible. The glue that holds the oxide to the tape can either go brittle, or sticky with age, depending on tape type. and if speedily spooled, can shed the oxide, with all it's attached information. For the really important tapes, there are specialist companies that can "bake" the tapes before playing them. But you might only get one shot at the transfer before they self destruct. (Shades of Mission Impossible there!).
3. I'm sure that there are companies that still make proper splicing tape. If possible, try to remake any edits, as the glue on splicing tape is more prone to "Bleeding".

Finally, try and store the tapes in a coolish dry atmosphere, the emphasis being on dry.
Finally, again. Assuming that you are going to transfer the material to a computer initially. Make a back-up to another hard drive (preferably 2 if the material is of real worth)
Good Luck. I would estimate thet you've got about 3 Years work ahead of you!
Regards Ralphie (Owner of lots of tapes!!!)