genrally i make my props from cardboard from the local supermarket on restock day as weather you pay for it or get it free after use, cardboard tends to be the same quality. although if you wanted something cardboard like, but nicer than cardboard look in to foam board. you can get it in larger ammounts from photoframe shops. it has 3 layers one foam layer inside and 2 white poster like layers on either site of that, but it can be expensive, 25inch square costs about £5.
sorry for the delayed reply been away from the site.
MCM Midlands Expo 2013
COSPLAYS FOR CON
Zero (Code Geass)