Milk Bottles

We recently found a couple of old half-gallon glass milk bottles at a local shop (for only $5 each!).  I don't know what it is, but the milk just tastes better in a glass bottle.  The glass definitely holds the cold better.

However, they didn't have any kind of lid...so, I've been thinking of how to make a lid that would fit.  I'd thought of some sort of fabric/vinyl circle with an elastic edge.  Sort of like a very small shower cap, but that was a bust.  I just couldn't get it small enough and snug enough at the same time.  So right now, it's just a little dust cap of a lid.  Does anyone know of any resources for milk bottle caps (new ones)?