There are some on the exchange that are pretty good. Or you could try out www.thesimsresource.com. Accounts are free and most of the downloads are too. --- "Never take life seriously. Nobody gets out alive anyway."
You can also get them from the sims 3 site. My sig has a link to the houses I've made, and one or two of them are mansions. --- http://www.thesims3.com/mypage/ankhinfinitus Go download my houses. They are the best.
I'm using the second one right now for my main family, but i switch back to the first every now and then. I've honestly haven't felt the need to DL anymore houses since i've had these added. --- "To name my greatest strength, I guess it would be my humility. Greatest weakness? It's possible that I'm a little too awesome. - President Obama