First off... who on Earth came up with the name Grommet? Whoever it was, it's cool.
I bought an Antec computer case a while back and the 120mm fan on the back was mounted with these cool plastic looking things instead of the normal fat metal screws with threads so thick you can't easily screw them into the case without a power drill. The "plastic looking things" are, in fact, referred to by Antec as "Silicone Fan Grommets."
Needless to say, I was opting for the Grommets as I hate messing with those fat metal screws; and I just really like the word Grommet.
Luckily when my fan died, the hardware kit that came with my case contained 4 extra Grommets I used on the replacement. I also noticed an internal mounting spot for another 120mm fan to help extract heat from the hard drive bays. I decided I wanted to mount a fan there as well, but knew I'd have to have Grommets there or I'd be in big trouble trying to get the screws to mount in that area.
I decided to try Googling for them; but realized I had no clue what they were called. I started with the phrase: 'antec fan mount screws' (without quotes) in hopes of finding them as a "related item."
I quickly gave up and headed to the official Antec website. They have a "Spare Parts" link that will allow you to buy the not-so-easy-to-find parts. It was there (and thank heaven most items have images next to them) that I found the Grommets. I never would have found that name on my own.
Searching for Grommets on the search engines after I discovered their "official" name didn't turn up many other options for where to purchase them.
So if you're looking for them, I'd recommend Antec. When I purchased mine, they cost $6.45 for a set of 4, but the shipping was free; so that was nice. If any of you out there know of more places to purchase Grommets, please leave a comment with a link so we can all be enlightened on where to find these very useful fan mounting tools.
You know what they say, "by small things shall great things come to pass." Since these are meant to help noise reduction with the fans, I'd consider that a great thing.