Whose idea was it to send the twelve spies into Canaan?

The idea to send the twelve spies came from the Israelites, not from God. Moses recounts that God told them to go up and possess the land, but the people, weak in faith, insisted on first learning what they faced. God condescended to their request.

Les explains that in Deuteronomy 1 Moses is repeating the history of Israel's journey from Mount Sinai to Kadesh Barnea. God had set the land before them and commanded them to go up and possess it. Moses asks whether God told them to send in twelve spies, and the answer is no. The idea came from the Israelites themselves. They backed off, were weak in faith, and forgot the power their God had manifested. They said they could not go in until they knew what they were up against, and God condescended to their request. Les notes that those Jews must have wished those twelve had never gone in, because their report caused Israel to become weak, afraid, and unwilling to go.

Deuteronomy 1:22

And ye came near unto me every one of you, and said, We will send men before us, and they shall search us out the land, and bring us word again by what way we must go up, and into what cities we shall come.

Whose idea was it to send the twelve spies into the Promised Land?

The sending of the twelve spies was Israel's idea, not God's. The people doubted God's promise to drive out their enemies and give them the land, so they asked Moses to send men to search it out. God graciously permitted it, but it was not His intention.