Tubeless, 26", rim brake? I've not done an extensive search, but I think it unlikely there'll be much of a selection, if any. Tubeless became popular about the same sort of time as disc brakes became the norm and 26" was falling out of fashion. It's a size that'll be around for a long time, due the the volume of bikes about, but I'd be surprised if there was anything new offered in that size till it comes back into fashion. There's a choice of tubeless 26" tyres, but they're most probably all MTB knobblies, so even if you found some rims, you might not find the right tyres to take advantage.
I must admit I'm tubeless curious, but it'll wait till I'm starting a project from scratch, it isn't much of an up-charge where swapping is