So you can look on Amazon, it's hard to find what your looking for though. Etsy is another good place but you need to buy in bulk I found to get free shipping or your paying an absurd amount for just 1 on shipping. I'd also suggest maybe talking to a jewelers shop to see if they have any empty rings or can obtain them as well!
check etsy and ebay
I don’t know if they have anything for breast milk, but Mene has 24k gold jewelry.
Make the resin part and the. Go to a jeweler to have it set, just like you would a stone or pendant of another sort.
Etsy
i wanted to make a breastmilk photo locket so i got a custom gold one made! most jewelers will do custom pieces if you contact them
I imagine a lot of sellers for real gold are going to be wholesale type sellers (meaning you’d have to buy in bulk). So I assume it would be cheaper to go through a company that specializes in making this sort of jewellery. Many offer pieces that are not “costume” jewellery (I assume you have an allergy or some other reason you can’t wear cheaper metals).