Get cheap plants. In my rearing tank I have a heart leaf philodendron and some other tropical plant with silvery leaves. A big 4" pot of either will run you about $1.50 at Home Depot or Lowe's in the houseplant section. Both are very hardy and can be grown from cuttings, so you don't have to use the whole plant. They also have the added bonus of being nice looking.
The heart leaf philodendron is a trailing plant, but can also climb and spread out. It is a fast grower, so will colonize the tank quickly, which is what you want in a quarantine tank. It is a very easy to recognize plant once you know what it looks like:
http://www.denverplants.com/foliage/html/phicord.htm
Ignore the watering information. Mine is growing like gangbusters and the soil never dries completely, of course.