diy matching separates???

So, I’m trying to not spend more than $400 for a dress that I will wear once. I’m pretty conservative as far as dressing. So I prefer high necklines and a not low back.
I’ve looked at some online stores. The China ones are scary. Etsy seems ok, but still I’d like to have the option to return it if it didn’t fit.
I was looking into separates, but most of the tops are like belly shirts. Also, I’m afraid if I don’t get the skirt and tip from the same place they wouldn’t match whites. Anybody have experience or advice??