What ARE the old books?
Internet. Alibris or websites. Search the book you want. Ask for anser book and the Mental math book
http://www.amazon.com/Saxon-Math-54-Incremental-Development/dp/1565770331/ref=pd_sim_14_4/184-4076035-2454506?ie=UTF8&refRID=1MN2JYZNXHZSP76ZH9BV