You might want the older versions for older Operating Systems; some people still use Win95, Win98, etc. and it may be a problem to install, say, WMP9, on Windows 98; it can be done, but it may be easier to install an earlier version.
Also, different versions can cause conflicts in other programs; e.g. Roxio EMC7.5 works OK with WMP10, Roxio EMC9 Suite doesn't.
It is interesting to note that MS offers a couple of security updates for WMP8, but not the player itself. Duh.
You should find it on an original XP disk, (before 2003) and I expect you could find it on those weird sites that we don't discuss.
I'll have to check my Father's disks from his acquisition of XP in 2001; if it's such a rarity, perhaps it's worth something.