I was browsing the Pottery Barn website when I found these mini skull vase fillers, but they are no longer available. I liked the slightly creepy look they gave inside the glass vases.
After a jaunt around WalMart, I found some miniature plastic skulls that had a similar realistic look to the Pottery Barn skulls.
I had a vase from the thrift store, and it only required one package of the WalMart skulls to fill to the brim (with 3 leftovers!) You could easily fill multiple vases for just a few dollars.
This has to be one of the easiest DIYs ever, for a classic creepy halloween decor.