Firefighters

Sheriffs

Sole proprietorships & partnerships

Privately-held corporations in non-political, non-heavily-regulated industries

Custodial banks (Schwab, Fidelity & TDA)

Credit unions

These may not be "institutions" in your sense, but I'm sympathetic to these & haven't seen corruption:

Mises Institute

Ayn Rand Institute

Federalist Society

Low Carb Down Under

Watts up with that

Gun Owners of America