Ivy and Rose is a good recommendation.
Take a look at Etsy and Ruby Lane too. But many target rings are actually reproduction settings. It can be hard to find an authentic one in good shape. Here are some options in budget:
https://www.rubylane.com/item/1426214-D427/Vintage-Rose-cut-diamond-target-ring?search=1
https://www.rubylane.com/item/1426214-7643/Antique-Edwardian-Diamond-Target-ring-circa?search=1
https://www.rubylane.com/item/10305...orian-target-ring-diamond-calibrated?search=1
Etsy has some too. Really depends on if you want a cluster style, a repro setting, authentic setting, etc.
Smaller center diamond but very nice IMO (negotiable price):
https://www.1stdibs.com/jewelry/rin...18-carat-white-gold-target-ring/id-j_3889413/
Have you tried Ivy & Rose? They have target settings and loose diamonds.