Ordinarily people would use picture frames as these are not popular however you can diy, that is if you can find the right thickness.Again can try to look for a paper seller or merchant. Another way is make your own mat board.You can stick thinner cardboard together and cover with coloured paper. Paper materials can be found in art stationary shop like Art friend in Bras Basar complex near national library or Kinokuniya bookshop Stationary section in Takashimiya shopping center orchard. For free stuff like discarded boxes at your void deck?
Edit: Come to think of it you could probably get a mat board from Artfriend!