It's really not that hard to do the processing at 100F. Tiberiustibz has alluded to the simplest and least expensive approach: a water bath. Take a dishpan or similar container and fill it with water that's a bit above the target temperature -- say, 102-104F. Put the bottles with your mixed C-41 chemicals in the water bath and wait for them to reach the target temperature. (If you're just using tap water to maintain the water bath, rather than a heating element of some sort, you may need to drain off some of the water and add more at a higher temperature to get everything to the target temperature.) You can also put the developing tank in the water bath to get it to the target temperature. (I use a pre-wet, too -- I pour in 100F water, then pour it out when I'm ready to start processing.) I start my water bath before I load the film in the tank. That gets the chemicals up into the 90F range by the time I've done loading the tank, minimizing the time I have to sit around waiting for the chemicals to warm up.
Even with temperature drift if you don't use a heating device, the drift will be pretty minor over the course of the 3:15 required by the C-41 developer, and that's the most temperature-critical step. (When I use tanks that permit using a thermometer during development, temperature readings don't change my more than 0.1-0.2F.) Kodak says that subsequent steps can be done within a range of 85-100F or 75-105F or some such -- I don't recall the precise range, but it's broad enough that my water bath has never drifted past the low end over the whole processing period.
You can of course get more complicated than this. I've mentioned heaters to keep the temperature at 100F; or you can buy dedicated processing hardware. The latter tends to be fairly expensive, at least by the standards of a Rubbermaid dishpan, so I continue to use a water bath and manual tank. I get consistent results, as measured by enlarging filter packs that don't vary my more than about 10cc when I use the same RA-4 paper and RA-4 developer.
