You can use a program called httrack to do this, but turns out wget is almost as good. I had a couple of old Joomla 1.5 websites that were now more of a security risk than anything and kept getting hacked, so I decided to turn them into static HTML sites using wget.
wget --limit-rate=200k --no-clobber --convert-links --random-wait -r -p -E -e robots=off -U mozilla http://www.website.com
That's it.
To download a sub-directory include "--no-parent" and make sure to add "/" at the end of the path(otherwise it'll run away)!
wget --no-clobber --convert-links --random-wait --no-parent -r -E -e robots=off -U mozilla https://website.co.uk/dinosaur_photos/diplodicus/