The Chinese supermarkets on Rómulo Betancourt and Chinatown might sell miso.
Samurai is sure to serve tempura. The less pricey Dragon House which has branches on Av. Duarte and Gustavo Mejia Ricart definitely has it on the menu.
You can use regular flour for tempura. I was taught it is the ice cold water in the recipe that makes the difference.
Can you get rice flour in the DR?
mkohn