Sourcing rubber grommets can be a tough task. The first hurdle is are you looking for standard rubber grommets off the shelf or do you need a custom rubber grommet.
If you are looking for standard rubber grommets you can probably find them form a major industrial distributor such as Grainger or a specialized distributor. I am sure you can find more from searching on Google.
If you need something custom the next hurdle is do you have enough quantity that a manufacturer will quote you with a decent minimum quantity. Often manufacturers will need thousands of parts to run a custom application.
You also need to know what type of material you need. Rubber grommets can be made from a variety of rubber compounds including, silicone, Buna-N, Viton® Neoprene and EPDM.
You will also need to specify size. Rubber grommets range in size from .094 to many inches in diameter. You need to know the ID and the OD of your part. Some grommets are Mil spec as well.