So it seems like knuckle rings have become SERIOUSLY popular and are popping up everywhere!
Knuckle rings are absolutely everywhere! With good reason too, these are brilliant. The ones I have currently are from the well known nail art website LadyQueen.com. The ring set comes with 5 rings in 5 different designs, The listing name is "Gorgeous Jewelry Set Crystal Set Shiny Gold Knuckle Rings 5PCS/Set" they're priced on the site for $3.38 USD & are well worth it, you get 5 different styles which is fantastic considering you can switch up the knuckle rings according to your regular rings.
These styles of rings are excellent because you can wear 1 by itself or multiple knuckle rings or even a couple of knuckle rings and a couple regular ring. They're pretty fun layering too! So have fun browsing their site they have lots of different items & lots of nail art items as well!