Define "make"; are you wanting to buy off the shelf items and make a costume, or physically craft the items of clothing?
For either, what is your budget (bearing in mind that off the shelf can be expensive, but making items might be just as expensive in different ways)?
Define "costume"; what kind of outfit are you thinking of wearing/what are you comfortable wearing? What are your friends going to wear?
How far down the rabbit hole do you need to go, i.e. are your friends committing to authentic looking clothing?Are they getting lots of accessories? Are they getting period footwear, or are they using sensible, worn-in footwear and trying to hide how modern the shoes/boots are with tied on fur or footwraps?
How long do you have until the event?
For off the shelf, you might want to look at:
Medieval clothing for men: shirts, pants and tunics for LARP & reenactment. Find garments at Mytholon now!
mytholon.com
https://dein-larp-shop.de/en-uk/collections/garments?filter.v.availability=1&sort_by=price-ascending (you can sign up for their newsletter and get 10% off)
Shop our curated Viking, Rus & medieval starter sets. Historically inspired costumes for men & women - perfect for LARP, reenactment & medieval fairs.
burgschneider.de
There are other shops, but delivery may be a factor; I'm more familiar with LARP shops in the UK, and have had shipping take several weeks when they've bought stock in from various European and Scandinavian countries.
If making your own outfit, that can be a lot of work (making sure you get the right kind of fabric, getting padding and/or inner lining as necessary), but you can get a lot of free patterns for cutting out fabric online, e.g.
It will make a lot of sense to ask your friends for pictures of what they'll be wearing, either as a description, reference pictures or literally them doing a trial fit of their outfits, then taking a selfie and sending you the pictures.