From a quick scan of military fuses it appears anyone can buy them from at least two sources, our old friends Littelfuse and Bussmann. I think the problem for audiophiles - aside from the obvious one, what they sound like - is that they are apparently all super fast blow fuses. If I’m not mistaken almost all aftermarket audiophile fuses are slow blow.
A good aftermarket fuse is slow blow or very long time lag, in basic design.
IIRC, it is SOC that came up with the best time lag element design?
To this day, Japanese audio gear comes equipped with SOC brand fuses, and I opened up a 1973-4 manufactured Yamaha CR-1000 receiver the other day, and inside of it were original...SOC fuses.
That's at least 43-45 years of Japanese audio companies using SOC fuses. There's a reason for that.
All that being as it may be, we know that we are probably invalidating our given house insurance when we put the wrong fuse type or value or both...in the given gear, just because it sounds better (to us). (I don't play that game, personally, I use my own fuse designs)