LensRentals.com is the place I prefer to rent from. In fact, a Nikon D4 is on its way to me for some weekend testing.
Use the tabs to select the type of gear you are looking for (“Zeiss Lenses”, “For Canon”, “For Nikon”, etc).
Note that the B&H pricing and stock status is updated every ~15 minutes or so, and color-coded appropriately (blue for in stock, red for out of stock). Bookmark the pages for quick access, as a handy way to find an item that goes out of stock quickly, or to scan a long list of items visually, instantly.