I don't have recommendations for non-marking stuff, but I definitely recommend Color Wonder (NOT the fingerpaints, for an 18 month old), as a non-marking. Better yet...I know at Target, and probably a lot of other places, Crayola makes a HUGE pad of paper--it has a built in handle at the top--and I use that for my toddler. I can seat him at the table, put the paper down, and the paper covers his entire reach. Of course, I give him washable crayons--we stay away from Markers--but this seems to work well for me. Or, sometimes, I give him an 8 1/2X11 sheet of paper, and I put packing paper beneath it (we moved 2 years ago and have literally thousands of sheets of packing paper).
Oh, and one huge, huge DO NOT: Crayola makes a 3 pack of markers intended for toddlers. They are round and shaped like animals...and they are AWFUL. I had them for my 2 year old, and he couldn't make them work--the tip is too small by comparison to the part you hold--and he kept holding onto the tip end, making it a HUGE mess. Even my five year old struggled with them!
Finally--I'm not sure where you intend to have your 18 month old color, but I'd recommend his high chair, if you're still using one. M