Well GM actually used rubber sleeves whenever they ran wiring through our doors, but the grommets that they used in other areas of the body, should still be available through your local Dealership... Just ask for whatever size you want, but the Secret most don't know, is that the sheetmetal grommets can be found at your Local Larger well stocked Hardware Store(Lowes/Home Depot/Etc).... A single grommet bought at the Hardware Store, will generally be not even half the price of what the Stealerships will charge you.....
If you are really set on the High Mark Up of a "OEM" Grommet that isn't available at your Stealership. then I would check with Year One/Etc (Chris might have some of the affordable ones though) .....