Kay Jewlers in any Mall.
Page of $100-250 Promise Rings:
http://www.kay.com/webapp/wcs/stores/servlet/CategoryProductsView?itemsPerPage=12&storeId=10101 &catalogId=10001&langId=-1&topCatId=15064&categoryId=20789&bcCatIds=15064.2 0789&Ne=1&Ntk=Category&Ntt=20789&N=%2B0&viewAll=fa lse&NPrice=6&NMetal=0
I bought this one for myself as a right hand ring, when it was on a really cheap sale. It looks crappy online, but in person it looks GORGEOUS, so go check it out!
http://www.kay.com/webapp/wcs/stores/servlet/product1|10101|10001|-1|21556600|15064|15064.20789
There are metal surroundings around the diamonds, so unless you look at it really close, the diamonds look like the entire round size, which would probably weigh 1/5 tdw if it were real like it looks.
Good luck! Cute idea
Also, if you are in the United States and have a Giant Eagle card you can get Kay Jewlers gift cards and get fuelperks!