So after hunting for AGES for the dress that Katherine was wearing in the masquerade episode and coming up with nada- someone else from coolspotters finally found it. Here it is:
Adrianna Papell Strapless dress for only 158.00.
Unfortunately, I can only seem to find it in that champaign color. BUT. I came across another site that has some pretty close alternatives.
Here's one- This is really close, but the lace is a different pattern, and the neckline is different. But still, this dress is incredibly cute and sexy, and only costs 68.60.
I'm still gonna keep my eyes out for a black version of the one I want.