where can i buy old coins and old paper money?

我記得新世紀廣場附近的廣場有得買
工

You can visit coins/stamp shops. Most of those shops are located at Mongkok and Sheung Wan. There are still a few located at Causeway Bay, Wanchai, ShauKeiWan etc.
You can also surf YAHOO, eBAY, REDDOT or GO2HK auction sites. For example, Yahoo auction site:
http://hk.auctions.yahoo.com/hk/20452-category.html?.r=1180366103