Little India is a good place for bus photo-taking.
Bendies: Try Bukit Timah Road, got 67, 851 and
960 bendies. Just need to get their shifts right. 
Double-deckers: Try Serangoon Road, near Tekka
Market, got 23, 64, 65 and 147 DDs 
857 and 980 with bendies on the weekends at Little india.