Here's one. I'm sure there are others. http://www.northernbrewer.com/shop/rubber-gasket-for-2l-growler.html
You're lucky that they are back in stock. I was looking for them in August, and everyplace was out . LHBS got me some in late Sept (probably form NB!)
That would be awesome, but I've never seen them on any McMaster Carr/homebrewing cross reference lists. My guess is that it's because they only have one fairly limited application, rather than general use.