The first one would be DPF.com, or
Diaper Pail Friends. The site owner was the first "main stream" formum somewhat similar to here. He expanded to have all kinds of business links, and even started to sell his own abdl stuff. Unfortunately when he passed away everything was shut down.
Daily Diapers eventually came about to fill the void Tommy left, but they have already made a firm line to be sex oriented only and do not welcome abs or dls anymore. I'm sure it's only a matter of time before they go out of business too.
One company that comes to mind is BKN. They had a somewhat brief run a few years back, and only made one model of diaper. It had a single tape per side just like Cushies, but the padding was much thicker and denser than Cushies. The plastic shell also had a geometric print very reminiscent of Huggies Snug N Dry, which you can see here.
I had a few at one point, and they almost became my favorite diaper then. Performance wise, capacity was near the top for its time, and I am a sucker for single tape diapers, but the cut was lower and tighter compared to Cushies, and the plastic they used had a tendency to stretch and tear after a while. The cost was a big thing, too, being imported from the UK, it made them expensive. I think that’s what really doomed the company.
It’s a shame they’re gone. I’d kill for some right now.
Curity used to make adult disposables (back in the old rectangular fan fold style) but they went back to baby products only.
There were a whole slew of manufacturers from the early days: Ambeze, GoAheads, Medalists. A lot of these were medical supply companies that may or may not still be around or even selling adult diapes under other names. I know Whitestone products is still at it.
Comco, makers of plastic pants, disappeared a decade or so again. They were my favorite.
I used to go to DPF quite a bit "back in the day". The chat rooms and forums appeared to be unregulated and not moderated, so yeah, the place gradually filled up with trolls and alsorts of people that crawled out from under their stones, some unsavoury characters indeed. Not a nice place at all. As for Tommy, from what I understand, but never seen official information is that he kinda portrait the whole AB/DL thing in bad light and did appear on TV a few times.