For Balian, we have the Organickidz stainless steel bottles. They come in 4, 7, and 9 oz., and they have both narrow and wide bottles. We are getting the 9 oz. wide bottles, however, we're going to use it as a sippy. (you can use all of the Avent sippy spouts with it or Thinkbaby sippy spouts as well)...and later on as a sport cup. http://www.organickidz.ca/ But we got ours from Babies R Us - one stop shopping for bottles, sippy spouts, trainer handles, etc. (and a bit of a price break)
For Bella, we have the Klean Kanteen - 12 oz, has all kinds of different top options, including a sippy cup (using the Avent sippy spout...which means you COULD use it with the Avent bottle nipples too...but 12 oz. seems a bit large for that...). Later on, when the sport top comes out with the organickidz stuff, we'll get those and Bella and Balian can share. The Klean Kanteen comes in lots of different fun, happy colors too! http://www.kleankanteen.com/