I did some searching for you and the average price of a bayonet mount ring is $7. Here are some websites where you can purchase what you need.
http://www.amazon.com/NEEWER%C2%AE-Bayonet-Mount-18-105-55-200mm/dp/B005HTP7MK
http://www.amazon.com/Neewer-Bayonet-Mount-18-105-55-200mm/dp/B005JQEFEM
http://www.dealextreme.com/p/designer-s-nokia-18-55mm-18-105mm-55-200mm-lens-button-cover-130802
Well, I have never needed to purchase my bayonet rings in bulk before; however there is a great website that should have just what you are looking for. I will post the link for you here - hope it helps.
I like to buy things for my camera from eBay. They usually have a selection of what I need at fair prices. I have found a bayonet for a Nikon camera and will post the link here for you too.
http://www.ebay.com/itm/NIKON-18-55-18-105-18-135-55-200-BAYONET-OEM-/140531628517
There is a great video on YouTube that will provide you with a thorough tutorial that will guide you through replacing your bayonet. I will post the link for you here.
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