Usually .so refers to a "which-ever version", as there are different versions of packages due to updates and bug fixes or distribution specific. So the request is for a package by a given name, version depends on which distribution and/or which version your package manager decides is needed based on dependencies of packages already installed.
Most times I find it is a package not installed and with an rpm distro I go to
http://rpm.pbone.net and put the name of the package with the .so and hit search to find which one is for my distribution.
Or one could just issue command:
yum install package.so
or
urpmi package.so
or
apt-get install package.so
Depending on which is your package manager.