For a few horizontal nipples the feed store is best, for five or so Amazon is good, for bigger lots, like ten use eBay. They all come from the same person, they live in Idaho and have the US distribution rights, so there is no difference in where you get them, at least in the US. Just do a search for horizontal nipple and it should come up for you. eBay is by far the cheapest per nipple, Amazon is a little more, they are most expensive at your local feed store.
Best to all and your birds,