While it's possible to buy ready-make grass tufts, it easy enough to make your own for the price of some PVA and static fibres if you own and electronic grass tool. The savings might even help to justify its price!
Take a piece of kitchen foil and blob some PVA around, aiming for a variety of different sized blobs, both round and lozenge-shaped. Put loads on, you can always keep some tufts handy for later use. Let the glue dry slightly before moving on to the next step.
Connect the earth clip for the grasser to the foil using the croc-clip. Apply the grass using a mix of fibres to avoid too regular a look to the finished tufts, then turn the sheet upside down and shake off the excess. This can be scooped up and put back in the hopper.
Leave the glue to dry overnight. It should now just peel away from the foil, although pair of tweezers are handy to pull it off.
To use, apply a very thin smear of PVA to the bottom of each tuft and then place it on the layout. I find gripping the fibres in the middle with tweezers the easiest way to do this. Pat them down lightly and leave to dry. Remember, the white PVA will turn clear and change the colour of the finished tuft.