I remember finding really great silver rings on ebay and then they stopped getting listed. Luckily, a regular over at the League of Heroes posted a link to Arroba Silver which is just what I originally bookmarked. The right GL ring is obviously vital if you are going to dress a Green Lantern; cool thing is that now there are other color Lanterns too and Arroba already has their logos available.
Also essential to rings would be a member of the Legion of Superheroes where rings and communicators are given when you are asked to join up. So for all the Phantom Girls and Beast Boys out there, you'll be able to find affordable LoSH rings now.