Plenty of stuff out there to build, tons of them are kits, tons are RTR, really depends on what you wanna spend and how much time you wanna put into it.
The Tamiya kits are usually you assemble every screw and nut, you put everything together and cut it from sheets.
Other stuff like Traxxas or HPI Racing are all RTR or fully assembled, so you just pull it out of the box, add a battery and run.
Then there's others where you buy the chassis, and add your own servo, receiver, ESC, motor, battery, and transmitter, and sometimes body (which you paint yourself) and run that. Tends to be a bit more pricy, but there's more fun stuff (servos and ESCs) and less not-fun stuff (bolts and nuts), but that depends on your viewpoint.
Superior Hobbies up on Semoran is awesome for land vehicles and boats. Can't recommend them enough. Go and look around at their stock.
Already wanna get a Baja 5sc Flux. >.> http://www.hpiracing.com/kitinfo/106258/