Katzung pharmacology (has an index section for DIs) and various databases. Lexicomp has descriptive monographs of the interaction and management (monitoring needed, discontinuation, switch to alternative, etc.). Epocrates DI system is ok in that it's fairly simple & easy to understand (compromised by lack of detail).
A really important aspect of the databases is to pay attention to the higher category DIs (more severe ones) and not so much to the trivial ones (CNS overload). Might want to cover the narrow TI drugs more (levothyroxine, warfarin, theophylline, digoxin, etc.).