Best place to get just a grommet... Is visit your local smoke shop. You get the day you get to store, so you can start smoking again quickly.
If you just want a grommet off the web the shipping is going to be retarded for just .50 piece for the most part usually a standard 6 to 10 buck shipping fee starting... But ya Ebay... You can get 3 Hose Grommets for buck with free shipping but you don't know the quality.
If you want some Shisha or something else to go with... Maybe check out www.hookah-shisha.com
or pretty much any other hookah web vendor.