Pirelli Heavy Duty Front Inner Tube 3.00-21 / 80/100-21 / 90/90-21 Pirelli inner tubes are a good heavy duty inner tube. Available in a multitude of sizes.

Michelin Ultra Heavy Duty Front Inner Tube 90/90-21 90/90-21" TR4 Valve 4.00mm Thick Fits Rear 80/100-21" and 90/90-21" Tyres.  

Michelin Heavy Duty Front Inner Tube Trial 2.75-21 2.75-21" Special Trial TR4 Valve Fits Front 2.75-21" Trials Tyres. Heavy Duty Reinforced Inner Tubes are 2.5mm thick. 

Michelin Heavy Duty Front Inner Tube 70/100-19 70/100-19" TR4 Valve 2.5mm Thick Fits Front 19" Tyres  

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