1986 Stumpjumper Sport Cog Replacement
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.