I just ordered everything for my daughters sweet 16 from Wish. Its cheap and pretty good quality. You have to give yourself 2-3 weeks at least to get it delivered but its worth the wait. Ill be odering from there for my babys 1st as well. Make sure you read reviews on each item and check sizes. Party City is good, Dollar Tree, too.

I like to make my own decorations because you can personalize them,try Pinterest they give you great ideas

What @valerie_jean for most of it! But if you are looking for super unique and detailed specifically for your daughter (like stuff with her name on it) I would recommend etsy! I got some truly unique pieces for my son's party. I loved it :)

Party city online! They have more choices

Dollar store , party city