1986 Stumpjumper Sport Cog Replacement Original Shimano Deore components all around.  Shimano MF Z012 5-cog freewheel.  Biggest cog is 30 teeth is worn out, chain skips,  and that cog needs to be replaced. Rest of cogs are ok.  Are compatible individual 30 teeth cogs still available as new parts?  Source?  If not, what's the recommended solution?
If the 30 is worn to the point its slipping and jumping, I would my guess the others must be due for replacement, along with the chain. 

Individual cogs tend to only be available in high quality gear, so a new cluster will probably cost less than a single cog. 

Replace the cluster and chain if you can afford it. If not, the cheapest way would be to get a beater from the tip, side of road etc, and take the 30/32 off that. Its rare that the largest cogs are worn out more than the others  (more teeth per rotation, lower torque) so even a really old bike is likely to have a useable large cog.