Well in that case they are part of the tank and not sold by mower shops
So you will need to remove them take them to an engineering supply shop and ask for "Two grommets like these in petrol proof Nitrile rubber please "
Or you can try to measure them if they are in a reasonable condition
Measure the size of the hole in the tank which is the outer ø and the tube ø which is the inner , the total outside diameter is not usually specified plus the total thickness then try your luck on line
Aircraft Spruce are good for a couple or McMaster Carr if you wand a packet of 20
Eastwoods also have a big selection but most of them will be neoprene which will go to jelly in a short time so make sure they are Nitrile or Vitton .
Silicone is also not petrol proof unless it specifically says "petrol Proof "
Grommets are funny sizes so if you order a 1/2" one the hole will be 0.010" to 0.020" undersize to get the tight seal. while the outer diameter will be the correct size .
Worst case would be to go to an auto store wit the tubes in your pocket, find a wiring grommet that fits double check it is tight in you tank then take that to an engineering supply company and ask for grommets this size in Nitrile or Vitton .
This mob is a manufacturer but if you find the size you want then ask nicely they will probably pop a couple of them to you in the post or give you the name of a retail outlet that stocks them
Note the measurements so measure your hole, tube & tank thickness according to the table