How can I replace the bayonet mount ring on my Nixon camera?

Check ebay they have many Nikon parts. Saw one just before Christmas. Ask part suppliers not average guy. They have broken lens an other parts on hand. Also check real camera shops that repair gear. Gotten parts that way also.

A bayonet mount allows lenses to be changed quickly and ensures they are locked into place

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.

http://www.alibaba.com/showroom/bayonet-adapter-ring.html

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

http://www.aliexpress.com/store/product/Freeshipping-BAYONET-MOUNT-RING-FOR-NIKON-18-105-55-200-MM-LENS-retail-wholesale/207456_477457118.html

That would depend on the particular screw mount. The most common screw mount was the M42, and one source for the appropriate adapter is
http://www.adorama.com/LNTMPU.html

You might want to call or email them and specify the camera on which you want to use the lens. Without knowing at least the brand of the camera, no one can tell you what adapter to use.

replace the Bayonet Mount Ring. i had to do it to my nikon 18-55 lens you can get the part on ebay (about $10.00).

If it's the VR lens:
http://uscamera.com/jaa79851-b37.htm

If it's not the VR lens:
http://uscamera.com/jaa79351-f.htm

As to whether you can replace it yourself, can you use a screwdriver and a soldering iron?

Probably screws that are on the part of the lens mount that touches the camera bayonet

little spring inside body behind mount has come of; open and repair; not difficult

Easy I purchased an adaptor ring to convert screw mount to "K"