Where do you get a sword in Harvest Moon DS?

Answer 1

When you have cleared the 1st mine you can activate the 2nd mine. Visit the excavation site as you did before and assist Carter with smashing the wall. The second mine is 255 floors deep, and you can only fall (at most) 10 floors at once. This mine has a lot of wild animals to fight off, but when you reach the bottom and clear away the enemies you will discover the Legendary Sword. You can pick it up if you have at least 1 of your tools at Mystril upgrade level (400%) in HMDS, but in HMDS CUTE you must have all of your tools at Mystril.

(In the English HMDS Keria gives you the sword. In the Japanese version it is just laying on the ground for you to collect.)

You must destroy the creatures on the 255th floor for the rock blocking the room with the sword to dissolve. You can't cheat by saving your game and then turning off your DS, expecting the creatures to all be gone when you reload. The creatures will be gone but the rock will also still be there.