The source code of a driver can be:
The Linux Kernel and Open Source drivers are maintained and developed by many different people around the world.
Closed Source drivers are developed and maintained by their creators since only they have access to the source code.
The source code for the drivers already integrated into the mainline kernel can be found here:
In OpenWRT, all kernel module package filenames begin with kmod-.
The modprobe command is not available in at least some firmware version of OpenWrt. Use insmod instead.