How (or where) do I find an airfoil optimized for low reynolds numbers (100,000~150,000)?
Try the HQ series from the UIUC Airfoil Data Site. That is HQ as in Helmut Quabeck (model airfoils), not Horstmann-Quast (those would be glider airfoils).

Another possible source is the website of Martin Hepperle.

Unfortunately, there is not one, but hundreds of airfoils optimized for the model aircraft range of Reynolds numbers. To be any more specific, I would need to know more about how you plan to use that model:


Will it be a glider with a low wing loading, trying to stay aloft as long as possible?
Will it be a fast glider for ridge racing?
Will it be a powered model for aerobatics?


Each case will need different airfoils.