Farmtek calls them Tomato Rollerplast Assembly:
http://www.farmtek.com/farm/supplies...13_110013.html
They are cheap (.95 each, .89 for 50-99, .75 for 100+), work great, and the spool has enough string to get through at least two years. Some of mine are going on year 3 and look like they could get to 4.