I believe readigrass is rye hay, so maybe you can find it there in Japan by looking for rye. We don't have readigrass where I live either, but I did find some bales of horse hay that were rye, and it was my rabbits absolute favorite hay ever.
There are a few things that have worked well for me to get new rabbits not used to hay, eating it(once dental issues are ruled out as the cause). The first thing was to offer almost unlimited pellets so they didn't starve, but rationed enough to give them a chance to get hungry enough to want to try some hay out . So what I did is I fed twice a day, and I would feed enough pellets each feeding that my bunnies would be getting enough food, but I limited it slightly so that they would run out of pellets about 3 hours before the next feeding. This would give just enough time for them to get hungry and have an incentive to try hay, but not get so hungry that it could lead to digestive issues.
The next thing I did for one non hay eating bun, is to start him out on a plain hay pellet. I'm not sure if you have something like this available or not though. All it is, is a livestock feeding pellet made only from hay and nothing else. I used a timothy/alfalfa pellet because most rabbits really like alfalfa and are likely to want to eat it, but I also wanted him getting used to the timothy. I got mine from a livestock feed store. Using this, you would replace part of the rabbit food pellets with some of the plain hay pellets, and still limit the amount of both types of pellets to run out the 3 hours before the next feeding, still giving a chance for your bun to get interested in trying the long stem hay. This helped provide more hay in my rabbits diet, even though he wouldn't eat the long stem hay at the start. Eventually he did start taking nibbles of the hay, and gradually he started eating more. As he did this I started reducing the amount of the plain hay pellets, until he was eating the long stem hay just fine and I could give a normal pellet ration to him.
http://standleeforage.com/product-formats/pellets
With another new bun, she was used to fresh grass so I started gathering fresh grass to give to her. Then I started mixing it with some hay, and gradually increased the amount of hay and decreased the amount of fresh grass, til she was eating the hay without any problems.
It can be a bit of work trying to figure out how to get a new bun eating hay, but sometimes it is just a matter of finding the right trick for that particular rabbit. Best of luck!