The gentleman from Vermont offered good advice -- about what I'd suggest.

Sometimes it's just cheaper to get a full-size ceramic stick, score it, and snap it off to the length you want.
Among other offerings, you can get the Lansky pocket triangular sharpening rods, 11-inch long fairly smooth 3/8" kitchen crock sticks, and a mondo ~1-1/4" thick ceramic stick at Smokey Mtn Knife Works. FWIW, the handles on the 3/8" ceramic rods have been pretty loosely glued on in my experience, so they're pretty easy to knock off the stick. I also like the Eze-Lap Sportsman diamond pocket rod. It is the size, shape, and has the pocket clip of an ordinary click-type ballpoint pen. Quite small & tidy.
http://eknifeworks.com/webapp/eCommerce/index.html
SMKW part numbers for sharpeners mentioned above.
1. Big John Super Stick Ceramic Rod - item SI1000 - $6.49 (1-1/4" thick)
2. Tennessee Big Stick Ceramic Rod - item SI1020 - $0.99 (3/8" thick)
3. Lansky® Dog Bone Sharpener - item G102604 - $1.99 (triangle pocket stone)
4. Eze-Lap® Sportman's D Shaped Shaft Diamond Sharpener with Grooves - item SI4 - $4.99
Otherwise, there are pocket-size diamond flat- or rod-shaped sharpeners that range in the $5-$35 range, depending on sizes and single- or multi-grits offered. Eze-Lap and DMT are the two most commonly seen brands, but there are others like Gerber, Hewlett, Schrade, and Smith & Wesson. In kitchen supply stores you may find names like Chefs Choice or Henckels diamond sharpeners, plus others.