I need an intuitive icon which represents "more" ITEMS (or "more items like this") (or other "similar" items to this item). I have space constraints on the screen, so I can't exactly use a label.
You could combine the ellipsis and a right arrow as follows - where the arrow is similarly styled to the ellipsis - same height lines as dot diameter, same curvature, same colour, same spacing.
 After Philip's comment below, I added the downward versions just to compare and contrast...
As far as I know, the "icon" that's used a lot for this purpose is a simple ellipsis (...), probably as a contraction of "More..." or "Show more..."
Also popular are all sorts of arrows pointing in the direction where the additional content will be loaded.
Plus signs are also occasionally used.
Overall, it seems to depend largely on context. If the content is loaded below the button, an arrow pointing in that direction may be appropriate.
If the loaded content is "more of the same," an ellipsis may be a logical indicator, but if it's somthing that extends the currently visible information, but is not necessarily similar in nature, a plus may be more appropriate.