It depends on your timescale. You could grab the last version of the offline Flash Player, and that would work until it doesn't. (e.g. you get an operating system update that breaks it.)
The way that people frequently deal with legacy software long-term is to take a snapshot of a working set of all the software required to make something run (the OS, the content, the offline Flash Player), and make a virtual machine out of it. You're then depending on the underlying virtualization technology to stay abreast of any changes in the OS or hardware that might prevent the legacy software from running.