Of course, all participants of a retro party should have an appropriate appearance, which means that you should take care of your outfit in advance to comply with the dress code of the event.
If it's a party in the style of the 80s of the last century, casual clothes are definitely not suitable for such a day. In a modern wardrobe, clothes are completely different, and you should look for something more appropriate to the moment.
You should visit the online second-hand store https://www.sonnechko.com/. Here you will find everything you need: used branded clothes and shoes of the past years at a cheap price.
The page of used ladies' fashion https://www.sonnechko.com/product-category/damen/ will offer branded women's clothing of different years of production. Need an evening dress? You'll get a unique piece for only a couple dozen euros! If you look at the used accessories page, you can create an amazing outfit for a party and feel amazing.
Complete the look with hair and makeup in the appropriate vintage style. It is worth studying the fashion magazines of those years to get the spirit of the time and look organic and natural.
Retro parties are very popular entertainment. It's like trying on the life of that time, virtually traveling to a bygone era. So you should prepare well to make the party authentic and atmospheric.
Of course, you can buy new modern clothes that look a bit like the old fashion. But it won't be real, it will look more like a bad fake.
If you have not yet decided what kind of retro look you want to try on, perhaps browsing the vintage fashion website will help you decide and find your desired outfit. At the same time, you can buy vintage shoes that will look harmonious with your clothes.