tocpcs:
I need the high impedance so that we don't take current from the circuit being monitored.
The ADC in the Arduino has a typical input resistance of 100Mohms, and a sample capacitor of around 6.5pF. So it disturbs the circuit being measured very little. Unless the input voltage is changing rapidly and you want to follow those changes, you may not need an amplifier at all.
If you want an inexpensive rail-to-rail op-amp in a non-smd package, here is a suggestion: http://uk.rs-online.com/web/p/op-amp/0403165/.