What's your budget?
I have a couple of engagement / wedding rings from previously discontinued relationships that were never worn, that I'd consider selling to you if you are interested (I have all certifications paperwork, receipts from the stores and online shops, etc...)
1. a $800 small diamond ring from Macy's which was to be a cheap engagement ring but we called it off (whew).
2. a $14k one point xx carat diamond ring from James Allen with platinum setting for a girl whom I thought would be my future wife but again ended up not working out.
I was a fool for spending so much on the diamond rings and I was just going to keep them in my safe as a reminder of my foolishness in past relationships, but if you are serious we can work something out. Either way no big deal... love is blind and the diamond rings are my reminders of that fact.
Honestly if I were you, I'd save the money and buy something more utilitarian, but if you are dead set on buying a diamond ring, you can pay retail prices on sites like James Allen, which I highly recommend, or buy my mistakes at a discount. :)
PS - this is a way of giving myself a face punch for blowing so much money on such frivolous items, but no matter, money comes and money goes... if you truly think the girl is worth it, money shouldn't be the top priority. Plus the purchases didn't hurt my financial situation much, as I am very close to FIRE and the purchases are a negligible expense.
PPS - OP if you are serious about buying a ring, and are interested in one of my rings (they are exquisite), PM me. I can then set up some sort of eBay listing for you to buy, so you are protected by eBay and PayPal, and I would have Seller protection to sell such an expensive item to you.