Paid for stuff is available here: https://www.123rf.com/ There is a lot of it and the quality is high. It is effectively £2.28 per image.
Free stuff (public domain) is available here: https://openclipart.org/
Stuff that is free to use but is not public domain i.e. requires you to credit the author and reference the licence is avaialble here: https://commons.wikimedia.org/wiki/Main_Page
There is also a small amount of public domain stuff here, though the quality is patchy.