I see that The Film Photography Project are now supplying 16mm b/w and colour film in 50ft Magazines. I'd guess there must be many, many of these old 16mm cameras in people's cupboards and attics. And although the years have probably taken their toll, I bet a large proportion are still in useable condition. And as the 50ft magazines were always more expensive than rolls of film on spools, it's likely they had less wear.
The FPP is US-based. So the costs of shipping may deter some in other countries. But it's good they are offering various film-stocks including Ektachrome. As the film is double-perforated I guess they have re-perfed some. They also emphasise the fact the magazines themselves can be temperamental due to their great age. I wonder if that happens in practice? Those I've used have worked OK. It was a great design. They enabled folks who could afford it (inc our old Queen !) as well as others on expeditions etc, to carry a compact movie camera everywhere and get quality results.
https://filmphotographystore.com/collections/movie-film/products/16mm-film-magazine-16-bundle-film-develop-scan