You could probably create this yourself? Get a 12-26/11-26 and then maybe an LBS will sell you the individual cogs you need.
For me I'd just need to get rid of the 12 and get a 18 cog. I wonder if the cassette lockring would screw on right on the 13?
For 105 5800, a 12-27 would be really nice, maybe a 13-26/13-27. (11 speed).
Last edited by zymphad; 05-20-14 at 01:05 PM.