I don't really care for digestives as a substitute. I think the texture is wrong. My fiance did some experimenting after I sent him a box of actual graham crackers, and he thought that Abernethy (sp?) biscuits were a closer fit. Not sure if those are available outside Scotland, though.
As others have suggested, though, Oreos are great, depending on the type/flavor cheesecake you're making. Also, gingersnaps (the really crunchy kind, not the chewy ones, obviously) are lovely, especially if you're making something seasonal, like an apple or pumpkin cheesecake.