Most computers built within the past decade have at least 6 USB ports. (I'm using a reconditioned Windows 7 machine, bought for under a hundred quid, but it's still got 2 USB ports on the front and 4 on the back). Are you sure that you've looked properly?
As you've suggested, most modern machines no longer have PS/2 sockets, which your old keyboard has.
Assuming that you can locate a spare USB socket, this should be ideal for your needs:
https://www.amazon.co.uk/Microsoft-Wired-Keyboard-600-Layout/dp/B001QSZ8IE
(That's not found from just a random bit of web-searching. It's the same keyboard as that which I'm typing on right now. Much as I hate to endorse anything with the word 'Microsoft' on it, I have to admit that it's one of the best keyboards I've ever used - and, yes, it has got fold-out legs at the rear to give it the slope you require).