The list of substitutes changes year to year.  Generally those thinking about joining the League or those that recently “retired” fill in.  As a sub, you could most likely play every week if so desired.  Your best bet is to know golfers that are interested in playing and ask them personally – that has the highest chance of success.  If you do not know anyone, you can visit the IMAgolfer site and find a list of subs (and their e-mail) under Golfers -> Directory -> Subs.  E-mail someone directly has a higher chance of success.  Last, and usually less successful, is to go to Tee-Times -> Schedule or Request a Sub.  This will mass e-mail everyone but the impersonal request only occasionally works.