Where can I find...
 

a bottom bracket lockring. Would my lbs have some lying around? And how much would one cost.

Less than $20.

An older bike shop is likely to have some laying in an parts bin in the back. Also try your local bike co-op, they deal with old Schwinn bottom brackets all day, so those thing's are a dime-a-dozen there.

I have one you can have for $18. Where do you live? Shipping might be a butch.

I would be surprised if an LBS didn't have one. If you get in good with them and they know they can count on your business they may throw it your way free. My LBS is pretty good about giving me small parts like that if they have them laying around (hub spacers, BB cups for projects, hose clamps etc.) Of course it gets expensive if they need to order things :D (just payed $20 to get replacement clamp bolts for my brake levers) not that they have a whole lot of leeway there.

My LBS stocks them with their SS/Fixed gear stuff. $1.99 for an FSA one.

Thanks