The easiest and most effective way to propagate them is through simple division. Of course you will need at least one parent plant of each variety that you would like to grow. If you shop around you might be able to find some 4″ inch pots at a fair price.
One of each variety is good for a start. I find that the best time of the year to divide them is in the spring, just before the new growth emerges. If you buy the stock plants in the early spring, you might be able to divide them right away. If you buy them at any other time of the year, just plant them in your garden or other suitable location, knowing that you are going to dig them up in a few months, or a year or so.
When spring arrives you can divide them at any time as long as they are not well into putting on new growth. The earlier the better. To divide them simply dig up the root mass and start dividing it into pieces. The divisions do not have to be to be very large. It’s difficult to describe, but as long as you have some roots, the new plant is likely to grow. If you have small young plants you can probably just tear the root mass apart with your hands, but if the root mass is very big then you are going to need some tools. You might need some heavy duty tools!
Last spring I divided several grass plants that had been in my landscape for a few years. When I dug out the root mass it was much larger and more dense than I expected. Using a very good digging spade and some real elbow power I was able to chop the root mass into quarters, and I replanted the quarters back into my landscape. That still left many clumps that I wanted to divide into very small plants that I could pot up in 2 quart containers. The root mass was too dense to tear apart with my hands, so I literally got a hammer and a 4″ wide mason’s chisel and chiseled off pieces. It worked and I now have a couple of hundred beautiful little grass plants in 2 quart containers.
Since then I have talked with a friend of mine who works for a large wholesale grower, and he told me that you never want to let an ornamental grass plant get that big if you intend to divide it. He said they plant small divisions in the field in the spring, and dig them up the following spring and divide them again. He assured me that if you get them just 12 months later, they can be easily torn apart by hand.