If it is really an old ring than you may want to contact
www.langantiques.com and pick their brain. If you want a diamond to match the period you may want to contact
www.rickshatz.com
If you are looking for "wholesale" be carefull someone does not sell you a dog. Try jamesallen.com or whiteflash.com to check prices or even to buy, very competitive and honest.
Good luck.