Inexpensive Costume Resources
Thrift stores such as Goodwill and Savers are wonderful resources for inexpensive costume pieces. You can find inexpensive leather belts, clothing (shirts, pants, dresses, etc...) that can be modified, sheets and curtains for inexpensive linen and cotton fabric, and a variety of odds and ends that could be great accessories for your costume.
Amazon is another good source for generic, inexpensive, base pieces of a costume, such as footwear, shirts and pants. Boot covers can be found for under $15, shirts can be found from $10-$30.
Thrift stores again are a great place to find useful and inexpensive options for decorating your camp. Some great things you can always find are wooden or metal bowls and plates, which help cut down on trash at the game and also add to the immersive atmosphere. Large plain sheets can easily be found for $1-$4 and are great for covering modern tents or modern camping chairs.
Clothes not Costumes: Layering - LARP Guide
How to Assemble a Great LARP Costume - Leaving Mundania
Creating a Good LRP Costume by Staying On-Brief - Larp Guide
Bleed: The Spillover Between Player and Character - NordicLARP.org
Creating your own Role Play - LARPing.org
How to build a Plasitidip Shield - Caliston Armory
How to make an Appliqued Tunic - Caliston Armory