There are SO MANY stuff sacks on amazon - just search and look for lots of good reviews. I bought (the wrong kind

) these last year
They're great but hubs didn't want dry bags, he wanted something softer, squishier, and more stuffable. I got this set of various
and my husband uses one of the shoe bags for his down coat.
We also have a teeny packable backpack he like to bring if he thinks he'll be taking the coat on/off when we're out during the day, but that's not available on amazon now - is a 20L water resistant bag, I'm sure there are other options online if that floats your boat.