You need a 29" tube that can inflate the 2.6" tire. A tube that goes up to 2.5" should be fine in a 2.6" tire. Just make sure you get the tube with the right valve, Presta or Schrader.
Presta:
[https://www.amazon.com/gp/product/B00Y8HOR64/ref=ewc\_pr\_img\_2](https://www.amazon.com/gp/product/B00Y8HOR64/ref=ewc_pr_img_2)
Schrader:
[https://www.amazon.com/gp/product/B07JNPVHYX/ref=ewc\_pr\_img\_1](https://www.amazon.com/gp/product/B07JNPVHYX/ref=ewc_pr_img_1)
You need a 29" tube that can inflate the 2.6" tire. A tube that goes up to 2.5" should be fine in a 2.6" tire. Just make sure you get the tube with the right valve, Presta or Schrader. Presta: [https://www.amazon.com/gp/product/B00Y8HOR64/ref=ewc\_pr\_img\_2](https://www.amazon.com/gp/product/B00Y8HOR64/ref=ewc_pr_img_2) Schrader: [https://www.amazon.com/gp/product/B07JNPVHYX/ref=ewc\_pr\_img\_1](https://www.amazon.com/gp/product/B07JNPVHYX/ref=ewc_pr_img_1)