glutinous rice
Short, opaque white rice that turns very sticky and chewy when cooked; despite the name, it is gluten-free.
Glutinous rice is reliably found in Asian grocery shops, increasingly in well-stocked supermarkets, and by mail order; it is usually soaked for a few hours before steaming. Make sure to buy whole grains rather than glutinous rice flour, which is a separate ingredient.
Substitute:
There is no true substitute: sushi or pudding rice gets sticky but never this chewy and elastic, so dishes that depend on that texture will not work.