Toh Rebars is a paid extension, not available for free through the official Extension Warehouse. However, there are a few ways to obtain it:
Click and select the downloaded Toh Rebars .rbz file. Toh Rebars V1.2.0 Extension For Sketchup Download
Quickly generate rebars for columns, beams, footings, and slabs. Compatibility: Toh Rebars is a paid extension, not available
What specific (foundations, beams, columns, or retaining walls) are you trying to model? Toh Rebars is a paid extension
You can find the official download links on the Toh Rebars Website or via their community links.