The Wagon bumper cover is not that hard to remove, easier with 2 people.
Remove a few plastic rivets either side, and underneath,
and unplug the reverse sensors connector that is on the left side [behind the left rear wheel],
And it should lift off the mounting brackets under the tail lights.
As for the round covers, they a plastic moldered/glued on the rear of the bumper, they normally shouldn't come away from the bumper cover, but nothing is impossible.
If you still have it, the best way to re-fix it back into place on the back of the bumper cover, is lightly rough the area up with sand paper, and get hold of some clear liquid nails [from Bunnings] and glue it back on,
Tip: when you re-fit it, place some masking tape on the outside and let the masking tape hold it in place, also place some masking tape on the rear of the sensor, to also hold it in position.
Wait for it to set, and re-fit the bumper cover, [taking care not to get liquid nails on the painted surface.
Otherwise, if its missing, the wreckers would be the "cheapest" best option, but you would have to try and colour match it, you could be lucky to find one the same colour.