It's 70 yen to send a postcard via airmail from Japan to US. You can purchase the stamps from post office counter or vending machines at post offices. - At the counter, if you just present the postcard and say "air mail to USA," then they will tell you it will be 70-yen, you pay, and they post it for you. In most places they will know what you want :)
- If you buy a stamp from a vending machine, just stick it on to the postcard, write "air mail" somewhere with blue or red ink, and post it into the red post box you find on street corners.