First, if you want to find the library where the *SBSD is stored, you can do DSPOBJD *ALL/xxxxx *SBSD
Where xxxx is your subsystem name.
Next, to start a subsystem, you do not need to know what library it is in because you can use *LIBL.
So, you can just enter STRSBS xxxxx
If it cannot be found in your *LIBL you will get an error, then you can use the previous command to find out where it is,