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:
cocktail dresses - sukienki koktajlowe
prom dresses - sukienki na studniówkę
eveningwear - sukienki wieczorowe
Some examples:
cocktail dresses:
zalando.pl/odziez-damska-sukienki-koktajlowe
allani.pl/wyszukaj/sukienki-koktajlowe?stores%5B%5D=125
prom dresses:
domodi.pl/odziez/odziez-damska/sukienki/sukienki-na-studniowke
allani.pl/wyszukaj/sukienki-na-studniowke?stores%5B%5D=125
sukienkimm.pl/sukienki/sukienki-na-studniowke
evening dresses:
domodi.pl/odziez/odziez-damska/sukienki/sukienki-wieczorowe
bonprix.pl/kategoria/846/sukienki-wieczorowe
evening dresses and ball gowns (suknie balowe):
evita.com.pl/produkty/suknie-wieczorowe
navona.pl/Suknie_balowe
sukienkimm.pl/sukienki/maxi
domodi.pl/odziez/odziez-damska/sukienki/sukienki-balowe
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.
