Flex pipe size for 2000 Chevy s10 4 cylinder?

My husband needs to replace the flex pipe in his truck. He wants to know which size he needs to do it himself. He took it to the shop today, and they told him that it would be cheaper for him to do it himself... So that's the plan! It is a 2000 Chevy S-10, 4 Cylinder. Also, any thoughts on where to purchase an affordable one online would be appreciated. Thank you in advance.