I have a tungsten carbide wedding ring that can't be resized. It fits
well in the summer but now that it's getting colder, it's beginning to
slip off. I almost lost the ring once. Is there some foam or bead
material that I can use to tighten the ring inconspicuously? And where
can I buy this? Thanks

My fingers shrink in the cold, too! I've had the same issue. Go to a
local jeweler and simply have a "ring guard" put on your ring. It's
adjustable so you can help your ring fit better in the winter and
adjust it according to the temperature. It's very inexpensive and
totally worth it. Hope this helps!

Sincerely,
Boquinha-ga

Over-the-counter ring guards are also available:

http://www.lgjewelers.com/proddetail.asp?prod=257%2D011

Another possible solution is a liquid plastic like this:

http://gallery.bcentral.com/GID4696604P3925595-11100-Ring-Guard-Solution.aspx