Bonnie Hunter sells her seam guide at her classes for $3. She doesn't mail them though. Walmart has them in the red colored plastic. I think by Dritz.
http://lh3.ggpht.com/--GC4n9zh_XA/UT...5B5%25255D.jpg
Here is a picture of how she handles the top loading bobbin seam guide.
http://lh3.ggpht.com/-3hmC4FGGbts/UC...5B9%25255D.jpg