Along with Mitch's technique, you might be able to use a coarser piece of hemp rope, unravelled and cut to length. It won't have the ears of grain on it, though, but depending on your scale, it might not matter.
If you're looking for a commercial product, you might want to have a look at Conrad Schulte's webshop, Fredericus Rex,
www.fredericus-rex.de. Conny has a number of products for reproducing vegetation, and even if you don't order from him, you can get an idea of what's available.
I'd also try model railroad hobby shops and websites.
Also, you might be able to use the diorama mats that are out there. Some of the makers who are members here at TF make products like that, their names escape me, though. You can also get synthetic fur cloth matting at a fabric story, that could give you the same effect as the grass diorama mats, probably much cheaper, too. I think I owe a tip of the hat to George (Warrior) for that tip. The mats can be painted. The major drawback, again, is that they don't reproduce the ears of grain that you might want to reproduce, if the scale is large enough to see them.
Thinking of Mitch's tip about using the bristles of a straw broom or brush, I think that you might actually find whole grasses at the craft stores like Michael's or AC Moore, so it might be worth a trip to one of those, if you have one in your area, or to your nearest arts & crafts store.
Hope that all helps, prosit!
Brad