I love those running boards! I usually call them skirts, or side skirts. Sadly, not very many people make them anymore. I did see some on Auto Accessory Super Store for over $800 for the pair!!
Definitely not worth it, imo.
Most of these skirts came on Custom Conversion trucks. Some good info on this thread:
https://www.gmt400.com/threads/where-can-i-find-these-oem-running-boards-body-kit.37371/
I actually found some at a salvage yard, they were an ECSB set off of a 94, custom conversion truck of course. I grabbed them for $20, I just had to snatch them before anyone else would, plus, the truck was crushed like a month later, so I'm glad I grabbed them. So if I were you, salvage yard is the best place to find them, or Craigslist.
Edit: added a link.