I don't usually buy such stuff on the internet so I don't know which are good ones, but I guess you could try general shopping sites where you can buy clothes like zalando.pl, domodi.pl or bonprix.pl.
You could also try to google it by yourself and see what you like:
If you are not a standard size girls, a custom made prom dress with your own size will be better. Check site: kissprom.co.uk. With free color and size custom service, you will get unique prom dresses just made for you at affordable price.