Hi-8 tape is also used in tape-based camcorders still, so hopefully it will still be around for some time. Hell, you can still buy Super8 film cartridges if you look hard enough, and that technology is what? 20+ years old at a minimum? I realise
Super8 is used for certain styles of movie making still so it has its niche, but I have a couple DA-38s I use for my main recording decks, and Im not too concerned.
Of course, I also hope to upgrade to
an Alesis HD-24 sometime in the next year or two... Why is it a concern? Do you have the deck(s) and wonder if you should upgrade now, or are you just acquiring one?
Id say in either case, you're fine for the time being, and I think the tapes will be readily available in bulk for some time. I think these recorders already have been relegated to the history books to a certain extent, as pro studios have probably mostly upgraded to HD for main use I think, but Hell, Tascam still makes a DTRS machine...
http://www.tascam.com/Products/DA-98HR.html
So, for now, I think we're safe