I believe these are just standard rubber grommets. You can find them easily in all sorts of sizes online. Sites such as this one in the UK provide a range of different sizes for different opening sizes, material thickness and cable diameter. The ones supplied from MikroTik are cut, you'll probably need to do the same unless terminating the cable yourself.
If the rubber seal is positioned at the bottom of the device, you can improvise with bicycle inner tube,
cut a piece of it and glue it, make a hole in the center and you have the rubber seal with a hole for the cables.