Hi bizhope,
Danny makes an excellent point. Buying directly from the manufacturer is the cheapest way to purchase goods because you are cutting out the middleman. They may require you to purchase in huge quantities, though, somewhere in the neighborhood of a thousand units - or more at a time. And often, manufacturers will only sell to registered businesses. Still, it's worth contacting them, see if you can negotiate an arrangement. Who knows, it might just work for you.
Anyway, I've listed some suppliers for the product you're interested in in case you need them.
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Good luck!