The first and safest place to look is the official SolidCAM website or their customer portal. SolidCAM maintains a massive library of standard post processors for the most common machines.
Once you have downloaded your free post processor (the .gpp and .vmid files), follow these steps to ensure safety: solidcam post processor download free
Which are you using (e.g., Fanuc Oi-MD, Siemens 840D)? Are you doing 3-axis, 4-axis, or 5-axis work? The first and safest place to look is