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My uncle used to make his own for his railway by buying a scrubbing brush with clumped bristles, putting a blob of glue near the base of each clump to grip them together, then cutting it away when the glue has set.
Tricksy. Gamers Grass from Great Escape Games do some lovely tufts, but only up to 12mm.
That would have been my suggestion. I don't know anyone who makes 20mm grass tufts.
Green Stuff World make 12mm Tuftshttp://www.greenstuffworld.com/en/68-grass-tufts….but they also make Tall Shrubbery tufts which are 4 cm longhttp://www.greenstuffworld.com/en/scenery-accessories/961-tall-shrubbery-light-green.htmlLots of colours http://www.greenstuffworld.com/en/106-shrubs-tufts