There is jewelry all over Disney World. Not sure there are any diamond rings there. Your best bet would be to buy at home and bring with you. . .hidden in your carry-on luggage.
I'm with nurse.darcy, get one from home and bring it with you. For something as important as an engagement ring you don't want to be relying on what limited selection there MAY be at WDW and personally as much as I love WDW and the main mouse I don't know if I'd be overly thrilled with a Mickey Mouse engagement ring (which is pretty much all I've seen there when looking at rings there)
I love diamonds and am always looking at rings. Sadly, Disney has not yet gone to that level. Please select your Princess the perfect promise ring at home and take it with you to WDW.
What a lovely thing to do by the way! I wish you both much happiness.
To get the most bang for your buck research and educating yourself about diamonds is key. I highly recommend this consumer website to do just that
pricescope.com
My sister's fiancé bought her ring at a jewelry store in their hotel. It's not what you'd call a traditional ring, but It commemorates the occasion. They also got the whole thing on film because the spot he picked was right in front of a photo pass photographer.
Good Luck!