Where can i buy a white (preferably a halter) dress w/ red polka dots?
The BEST things I could find..
http://www.modcloth.com/store/ModCloth/Womens/Dresses/Autumn+Overlook+Dress
http://www.modcloth.com/store/ModCloth/Womens/Dresses/Too+Much+Fun+Dress
http://www.modcloth.com/store/ModCloth/Womens/Dresses/Hot+Dotty+Dress
http://www.modcloth.com/store/ModCloth/Womens/Dresses/Share+a+Scone+Dress
http://www.modcloth.com/store/ModCloth/Womens/Dresses/Homeroom+Sweetheart+Dress
http://www.modcloth.com/store/ModCloth/Valentines+Day+2010/Cherry+Valence+Dress
http://www.modcloth.com/store/ModCloth/Womens/Dresses/All+Shook+Up+Dress
Couldn't find EXACTLY what you were looking for, but a lot of these do come pretty close 
you should look all over modcloth, they have tons of retro stuff there. Good luck.