Those are mostly custom built trucks, however they are based on existing rigs. I am assuming you are talking about the trucks in the video. Those Skyjacker crawlers are basically new and improved STOMPER 4x4's for the 21st century
The best out of the box crawler I can think of would be the
Tamiya TXT-1 you wouldn't really have to do anything but lock your diffs. I'm building up a Clod Buster wich has a dated chassis design (solid axles though and very low cg.) I've ordered a custom chassis with 4 link susp to build this truck into a serious crawler.
Most of the guys I crawl with use the IFS/IRS trucks. Traxxas E&T-Maxx trucks as well as the HPI Savage. They have made them work pretty good, but I'm hoping to give the "upset" with my solid axle truck.
The best way to get into this is to find a Local Hobby Shop (LHS) that sponsors/runs crawling events or other races. Or if you are looking to do this on your own you can find online shops like Tower Hobbies.
There are some good forums with tons of info on these trucks, rock crawling, and other events.
(Another good Forum)
(This is a retailer with lots of good stuff for the popular trucks).