They exist but are expensive. The stock unit, if in good shape works fine. A cheap Modine OEM replacement unit (from Napa or Autozone)works fine as well. The stock units will plug up after several (5-7) years and should be replaced.
FYI, on my G-Prod Civic (10:1 CR), I ran 180 all day long with a stock T-stat and stock radiator. I put in a brand new T-stat and it worked fine at the June Sprints, but a few days later it stuck closed while I was running the engine in the driveway (lucky catch!). So I decided to knock the guts out of the T-stat to act as a blanking sleeve. After that modification, I couldn't get the water hotter than 160 degrees at the Indy National two weeks ago in hot, muggy Indy weather! I will be replacing the sleeve with a smaller hole to choke off the flow to get up to 170 degrees. I run plain water + water wetter.
[This message has been edited by Greg Gauper (edited July 12, 2004).]