Windows 8 was much reviled, with most people preferring to stick with Windows 7 (and then to move onto Windows 10 later). So I've absolutely no idea why anyone should want or need a Windows 8 disc.
However it's easy enough to get hold of a disc, e.g.
https://www.ebay.co.uk/itm/Windows-8-1-Pro-DVD-HP/153966975729?hash=item23d9259af1:g:6UwAAOSwkexe4NdA
but a disc alone is of no use to anyone unless they've got a licence key to go with it.
What, exactly, are you trying to do, Henry?