Wedding date dress help

My boyfriend asked me to go to a friends wedding woth him. Its a destination wedding in Lithuania so its a big deal to me. My bf and his friends are much more...affluent financially than i am and this is a formal wedding.

Does anyone have suggestions for where i can get a really nice dress? Ive never been to a formal affair and these guys get all decked out. I prefer trying things on rather than ordering online, if anyone knows an actual store i could go to (ny)

Im really excited that he asked me to go and i want to look amazing. Pleaseeer help