It is good to see your enthusiasm in breeding budgies. Welcome. I use sawdust bought from a stock food place and mix it with unprocessed bran. The nest boxes are cleaned out every week.
I would like to suggest you buy a book on breeding budgies and there are plenty on ebay, especiallly any written by Cyril Rogers, this book is my budgie bible. Another good book is written by John Scoble.