One of my colleagues faced a situation where he had installed a plugin assembly into database but was not able to locate its dll, and now he wanted to deploy it in some other server.
These are the steps he followed
Get the encoded base 64 string representation of the plugin from PluginAssemblyBase table.
Use that content and get the dll from the following site.
Set decode and export to binary file option there with filename having extension as .dll
Open the dll in the reflector tool and use the source code to build that assembly.