Try streetsideauto.com. I'm not sure if they're cheap but they have a few options for floor and cargo mats. I'm looking into the weathertech cargo mat for the rear. It has cutouts for the third row seats which make them very versatile. Not sure what you're looking for in add on cup holders. Don't forget, this isn't a minvan
You might try cruising the regular Tahoe forums, most of the stuff will work (except the floor mats). Let us know what you find out there.