You can find loads of them on ebay. The best way to go is either buy cheap ones in various weights so you can decide which weight works best for you, before you spend money on expensive ones that might be the wrong weight for you. Or, you can buy a weight tuning set. The lighter weight will allow the engine to rev more before going up through the gears, heavier rollers will go up through the gears sooner, requiring more torque from the engine, these little engines don't have very much of that.
I am using dr. Pulley sliders, they are supposed to perform better and last longer.