muchmore also make some, they come in boxes of 20. We just got 3 boxes for our track
http://www.rctech.net/forum/5274289-post10237.html
They are low enough not to launch the cars like roadrail domes, but high enough that you are bumped well off line if you hit them. You can also clamber across them safely if you cut a hairpin bit you lose a heap of time
Edit: forgot to mention, they need to be glued down as this video of us testing them proves
http://au.youtube.com/watch?v=XEV6PAsuo5A