Nipples for Nuby Natural Nurser bottles?

Anybody know of any nipples that are interchangeable with the Nuby nipples? I bought a box of several different bottle brands and am looking for nipples for each brand. So far, the nuby are the most difficult to find- the closest Target is about an hour away and Amazon has poor selection of flows. The bottles look very similar to Tommee Tippee bottles so I might try to interchange the nipples but it's just a guess as of now. Anyone able to suggest a different nipple to use in the Nuby bottles?