Where can I get a BB lockring?
 

All I need is a simple free cup side BB lockring.

If you can't get one locally from your LBS you can get one on line from Harris Cyclery .

Umm. Those are not BB lockrings. Those are lockrings for fixed gear rear hubs. There IS a difference.
Lockrings for fixed gear hubs are smaller, and have reverse threads.
Just ask your LBS for a used one, I got one at my LBS for $.50.

My bad! I knew that, but I could swear that there used to be some BB lockrings on that page. For the people who use conventional hubs for fixed gears, they typically use BB lockrings on the freewheel threads with locktite to prevent the cog from unscrewing.

Sorry! :o

Try Loose Screws. I got one from them.

All you have to know is your threading and diameter, either English 1.37"x24TPI, French/Swiss 35x1mm, or Italian 36mmx24TPI. (I have yet to figure out where they got that weird hybrid metric-English Italian BB standard!) I may even have a spare in my junk box. Otherwise, I concur with the recommendations of Harris Cyclery and Loose Screws, my two favorite on-line bike parts suppliers.