How do I unlock additional trading carts for the Palico trader?

All farm upgrades come from Prowler village quests.

• Jurassic Fungologist (2★) unlocks the mushroom trade.
• Bug Reports (3★) unlocks the insect trade.
• Marshlands Fishing? (4★) unlocks the fish trade.
• Kut-Ku Earbreaker (5★) unlocks the second cart.
• Surrounded by Blue and Green (6★) unlocks the third cart.

Source: Kiranico. All of the farm upgrade quests are from Neko (Means "Cat").

You can additionally unlock everything by gaining HR from the hub. I don't have the exact numbers though on when things unlock. I believe I unlocked the second cart along with all the locations when I hit HR 4.