This part is called a Crank. You can buy cranksets but cannot buy a singular crank. There are many companies that sell cranks, the one on the Marlin 7 would be a Shimano one, so it would be best to buy a Shimano Deore Crankset or similar from you local bike shop or online from anywhere around $80 up to over $300 (AUD). But you could get one from other companies like Sram, SR Suntour, Raceface, Hope, and Truvativ. It must also have the correct amount of chainrings and must fit into the Bottom Bracket.