These 18-cell tall inserts are big enough and less expensive
than individual 3" or 4" pots:
http://www.novoselenterprises.com/pr....asp?ID=593160
They are made of a very thin vacuum-formed plastic, and picking up the whole insert full of potting mix and seedlings
without a web flat under it is precarious at best. The plastic
is thin enough that they are easily cut into individual pots,
strips of three, 6-packs, or 9-packs with scissors. Like Jack said,
reusing them always has some attrition, because they are so
thin. They develop splits up the sides, etc. If you are selling plants
in them, though, they are a lot cheaper per plant than individual
pots.