Michelin Ultra Heavy Duty Rear Inner Tube 140/80/18 140-80-18" TR4 Valve 4.00mm Thick Fits Rear 140/80-18" Tyres.  

Pirelli Heavy Duty Rear Inner Tube 140/80-18 Pirelli inner tubes are a good heavy duty inner tube. Available in a multitude of sizes.

Michelin Heavy Duty Rear Inner Tube 80/100-12 2.50 / 80/100-12 TR4 Valve Fits Both Front Or Rear 12" Tyres  

Michelin Heavy Duty Rear Inner Tube 100/110/100-18 100/110/100-18" TR4 Valve Fits Rear 100/100-18", 110/100-18", 120/90-18" and 130/80-18" Tyres.  

Michelin Heavy Duty Front Inner Tube 80/100-21 80/100-21" TR4 Valve Fits Front 2.50-21", 2.75-21", 3.00-21", 80/90-21", 90/90-21", 80/100-21" and 90/100-21" Tyres. Heavy Duty Reinforced Inner...

Michelin Ultra Heavy Duty Rear Inner Tube 100/110-100-18 100/100-100-18" TR4 Valve 4.00mm Thick Fits Rear 100/100-18", 110/100-18", 120/90-18" and 130/80-18" Tyres.