Why is this ammunition so difficult to find and so expensive when you do find it? The Type 99 rifles seem to be fairly plentiful, so demand for ammo must be there. What's the deal? Why does Prvi Partizan think it's not worth producing?

I'd love to get an Arisaka, but not when it costs $1.50/round to shoot. Just passed on another nice Type 99 because I don't want something I can't shoot.

A Friend uses 30 06 brass to make 7.7X58 Jap ammo.  There are YouTube videos.   Here is a post from another site I copied.  Might help, reloading is definitely the cheapest route.

Yes, you can convert the 30-06 (7.62x63) case to the 7.7x58 Japanese case by just running the 30-06 case into the 7.7x58 sizing die and trimming to length.
Back when I was doing so, I always annealed the 30-06 cases before resizing, as the cases lasted a lot longer before they cracked if they were annealed.
I used these converted 30-06 cases exclusivelly from the early 1970's thru the late 1980's, and I still have some of them that are still going strong today.
Once boxer primed 8mm mauser (7.92x57) became more readly available in the mid 1980's, I switched over to using 8mm Mauser brass to make the 7.7x58mm brass.
Just run the 8mm brass into the 7.7 sizing die and you are good to go.
Both the 30-06 and 8mm brass is a bit smaller in diameter at the base, but it will not hurt.
I still hand load 100% of the 7.7 Japanese ammo I shoot, and I shoot a lot of it in both my rifles and machine guns.
Today Privi (imported by Graf's) is making good 7.7x58 brass, and that is the way to go, but you can still make 7.7 brass if you want to.

https://www.grafs.com/retail/catalog/search?keywords=7.7X58+brass

It's a shame that reloading is basically the only option. It seems like a commercial offering in the $12-15/box price range would do quite well on the market