Amazon.com : Ultraverse Bike Inner Tube for 26 X 1.75/1.95/2.10/2.125 inch Bicycle tire Sizes with Presta Valve - Rubber Tubes for Mountain and Trail, Cruisers, MTB, and All Terrain Bikes – Set
Continental MTB 27.5 Inch Tube at BikeTiresDirect
A $38 Tube? The Tubolito Tubo MTB Tube Might be Worth it.