our petsmart used to have a great selection of live plants, then they got rid of the plant tanks. now they have just one narrow end cap of those plants in plastic tubes. they are really expensive and most look nearly dead.
the bags sound like a decent alternative if you don't have a lfs that carries plants....though i'd still rather get plants from a LFS or just have them shipped from a reputable source.
petco has a mix of the plants in tubes and a tank of plants. though the tanks are usually full of non aquatic plants...go figure.