There's three Den-O comics I know of:
A kiddie magazine called Shôgakko Ichinensei (Elementary School First-Grader) ran a comic version when Den-O was actually on TV, though I haven't read it and it hasn't been collected to my knowledge, so you'd have to look for back issues from 2007-2008. Going by the name of the magazine, I'd guess it's
really simple and likely little more than "Den-O fights a guy" every month.
Telemanga Heroes, aimed at a slightly older crowd, had comic adaptations of Climax Deka, Final Countdown, and Chô Den-O & Decade, and those are pretty good with some great art. All of those have been collected in volumes called "Super Heroes" (the Sentai movie comics are generally included as well.) I think only the latest one (which doesn't have a Rider comic) is still in print, but I've seen the back issues on Amazon.
The third "Chô Hero File" from Terebikun has a one-shot comic that's trying to look like Ishinomori drew it, though I think he would have done the Imagin better. There is a weird unnamed evil Rider with some kind of lighting bolt thing going on.
This is the book though it's harder to find than the first two in the series since it has a special sound FX CD too.