Well, I don't know too much about what might be on offer in Kendal, but you could try the Booths store in the town centre. Our local Booths has a little Japanese section with sushi kits (rice, bamboo mats etc..) and other stuff - even our little Tesco stocks the sushi kit. Check out the international foods section.
For a bigger choice, you are probably going to have to go to Chinatown in Manchester. There are lots of East Asian supermarkets and restaurants. Chinatown is in the city centre, close to all the main train stations. Go and make a day of it!
There is also Yo Sushi restaurant in central Manchester and central Liverpool, which are both good. Try their website for special offers.