Is not a good term to use with a CMOS device input, as the input is primarily a capacitor, with infinite resistance (impedance at DC). IBIS models are used to represent the diode input protection structure, any series inductance, etc.
Worst case leakage will dominate the impedance at low frequencies. The input capacitance will dominate the impedance at high frequencies. Leakage is in the data sheet.
Finally, any weak pullup, or pulldown adds a resistance to the input (also in the data seet).