Posts at 25.4 mm aren't bad to have around. I've beer-can shimmed one to ~26.0 without trouble and used it that way for years.
I had another in my bin that I thought would fit my Ross MTB (Ishiwata 024 tubing, it turns out). I brought it on a trip from Europe to visit my family - turns out the bike took 27.0 mm, so was much higher quality than I thought! I rode it with the aluminum post + external clamp and returned the next trip with a Cane Creek shim, which was nicely done in aluminum for the price. I had thought the post would fit my Finnish Helkama city bike to replace the galvanized (!) post it came with, but it turned out to be 23.0 mm with a nicely done plastic shim to 25.0 mm. I probably should have just honed the tube out...
I also had a Canadian-made late '80s Peugeot with HLE tubing that took a 23.8 mm post (15/16"!). Luckily the clamp diameter was the conventional diameter, so I was able to reuse the post with a clamp that I got from an LBS for a couple bucks. That's a size that's not so easy to find.