Recommend me a copper disc Rondeau / Stockpot
I'm spoiled by my Demeyere Atlantis lineup (grabbed these much more than any in my lineup).
In need of a 15" Rondeau and a 22-26 Qt Stockpot - copper disc bottom.
The closest thing I can find is the Sitram catering line.
Anything else on the market that fits the specs
I have seen this very occasionally with my Sitram Catering pots / pans, but mostly with a small pan on a large burner on full blast. With such a large pot, I don't think you'll have a problem under most conditions unless you're using a very unusual heating element. And, for any conditions where you're especially worried, I think just reducing the heat slightly should help.
All my pots and pans are the Sitram Catering line. I picked them up piece-by-piece at Homegoods several years ago. I have a 12" Rondeau (I paid $14.00 for it) and a 14" Stockpot ($11.00). I had to buy the lids online. They were about $30.00 each. I haven't seen it Homegoods since and I do look every time I go in.
I use it every day and I like it a lot. Highly recommended!
The only idea for you I have is vintage hotel-grade, tinned copper. eBay, Craigslist, and scrounging are about your only hopes in the US. You might try Hammersmith in Brooklyn or the French Copper Studio to see it they can do you a one-off.
There are deals to be found. I got an Elkington 10 Imperial Gallon stocker for $185.