Can't help with ink pad but I just purchased "finger paint" or non-toxic paint (I used some from a pottery shop as I wanted DD's footprint in a frame when she was two days old.
We have a lovely frame with a pink baby foot in it and her baby book has another (as I had enough paint we did her foot in a few places). It was easy to put on and to clean off!
Places like Bumps to Babes sell lots of coloured non-toxic paints.