There's a standard Raleigh type bag that attaches by straps though the saddle loops and a third strap around the seat post. The bags are made of an indestructible vinyl, but the straps are either plastic or leather, both of which are perishable and, by now, either at or beyond their useful life. Replace the straps and you're good to go. These bags someone's require an internal stiffener, too.
I found one of those on a Raleigh Weekender. They're actually very nice. It's currently on my Gazelle:
