My family got me the Apple Thunderbolt Cable (2m) from Apple Store at IFC, Hong Kong when they are there for a holiday. It is retailing for HK$288 in Hong Kong and S$48 in Singapore. Due to the weakening Singapore Dollar against the Hong Kong Dollar, it is about the same price after currency conversion. It used to be slightly cheaper.
You can connect your MacBook Air to your iMac using this cable and this will turn your iMac into a secondary monitor.