Rebuild a cassette?
The KMC chain on my 2008 Giant FCR3 has about 2900 miles and measures 1/16th of an inch over 12 inches even though it is frequently cleaned and lubricated. I decided to replace it but found that the new chain skipped under hard pedaling on two cogs. Examination of the original Sram PG830 cassette showed wear on the two cogs that are used most. In the meantime, I have reinstalled the worn chain, which does not skip, and ordered a replacement Nashbar cassette. I am considering rebuilding the old cassette as a backup. Does replacing the two cogs that are worn on the old cassette make sense? Is there a good source for individual cogs?
There is no good source for individual cogs. Shimano tends to offer only the smallest cogs (11, 12, or 13, designed as top level cogs) individually but I've never seen SRAM do the same. Campy seems to offer individual cogs but at the prices I've seen, you'd be better off buying a whole new cassette. Your best bet would be finding someone with a similar worn out cassette but with different worn cogs. I have a few old cassettes where only a few cogs are worn. They'll some day get used again when I come across a bike with difference cogs worn out.