Not that many shops in Japan always carry a full range of parts on the shelf, because most parts companies can ship out any part you want in a day or two at the most, so it's not worth them wasting the floor space or money.
The best way to do things is to print out a page from Defi's website before you go and circle the parts numbers of the ones you want.
If there are pictures on the page you can point at, that helps too, since the people in the shop will usually be worried about making sure you're ordering the parts you want.
After that, pay them in cash (which often gets a discount) and have the parts delivered to the hotel you're staying in. They should arrive in a couple of days.