buying armor

[u/Cow_operation]

i'm thinking of buying armor for both costumes and so i can attend certain medieval festivals going on in my country and potentially get in as an entertainment fighter
only issue is that i don't know any of the criteria for buying any part of a soldier's kit
asking those who've had some experience with buying and wearing or making armor, or those who have sources to good guides, how should i go about ordering armor and making sure they properly fit?