i got mine online from millybridal (china) they also have a ebay shop, cinderella bridal or something. i got mine custom made, it was the design of one dress and beading of another. it fit perfectly except it was a bit long but i added an inch or so as i was scared of it being too short!
i told them i was getting married a month before i really was just in case there was a hold up. at times they didnt reply to emails for a week or so but it all turned out good.
if you go along this path i reccomend you do go to a bridal shop or 2 to try on some dresses to find one you like/looks good on you. i couldnt imagine anything worse than getting your dress and finding it doesnt suit you