Loose rollers, wheels and axles for rebuilding a carriage you already have. Bored rollers with brass, stainless and polycarbonate axles, in acetal, nylon, brass and stainless steel. Cheaper than replacing the whole carriage when the housing is sound and only the wheel has worn. Match the roller diameter, wheel width and axle bore to the part you removed.
Not sure which one? See the Sliding Door Roller & Carriage Buying Guide, look up a part number in the roller part number finder, or browse all rollers and carriages.