Try idealist.org
But mostly there's not enough $ in non-profits to pay a s/w dev.
Another approach is to work hard, retire early, and then do the work pro-bono. I have been doing s/w for non-profits since I retired at 51 20 years ago.
Or just do both at the same time if possible. When I started my daytime job eight years ago, I managed to negotiate a four-day-week. Since then I used the fifth day to do freelancing in projects I find interesting and over the years I've shifted to use it mostly to do voluntary IT work for a local non-profit.