If you know of a reliable/secure site which offers such, please advise.
Try
this link for current version of Chrome (v64.0.3282.140 as of today) (67.9 MB). It won’t open/play/interpret .swf files though (I tried it), and even handles them like newly download files, with a security warning.
There is a stand-alone Flash player app that does work on my test .swf files:
Adobe Flash Player App 28.0.0.137 for OS X 10.9 or later (16.9 MB).
As Flash is deprecated, highly insecure, and at end-of-life (no further development), you should consider tossing the files, or converting them. The era of Shockwave Flash is over. Playback will get more problematic in the future.