Thanks for the image. It really helps. Now, I don't know about finding that particular dress for Sims 2, but you could try looking
here. There are some lovely Victorian style (which is the fashion era the Colonial dresses fit into) gowns there.
ETA: An image is *always* awesome (hosted on your own image server of course). If we can see what your looking for right in the first post, we know exactly what you want and what to direct you.

Or, what you did is fine.