The findPackage package helps you find the sos package.
Currently there isn't a release on CRAN.
You can, however, download the zip ball or tar ball, decompress and run R CMD INSTALL on it, or use the devtools package to install the development version:
## Make sure your current packages are up to date
update.packages()
## devtools is required
library(devtools)
install_github("findPackage", "Dasonk")