No specific suggestions for the DAC. Take a look at Digikey or Mouser. But before selecting a board you have to fully understand what you're trying to simulate and I don't have the feeling that's the case for you.
You're missing a very important part here: the pH sensor amplifier. You need this due to the very high output impedance of the probe, connect the probe directly to A0 and you'll always read 0V (or the offset voltage, if applied). This amplifier will normally also add an offset to the probe's output, so you can read with with a normal Ardruino ADC (which can do only positive voltages). You can simulate that with the DAC of course as well. Just set your pH=7 level at that offset.
Isn't it easier (and closer to reality) to just get yourself a pH probe + amplifier board?