afni.nimh.nih.gov

That was a mistake.

I just committed a change to InstaCalc to allow you to do this interactively as well -- instead of "Dataset: Value", choose "Dataset: Pvalu" from the menu item just to the right of the symbol name ("A", "B", etc.). And then choose the sub-brick index that you want converted on-the-fly. Presumably the expression would just be the letter "a" in such a case (although "-log10(a)" might also be useful).