They are basically large blocks of metal with cuts to increase surface area. Much more would have broken if it did, I imagine your fan just wore out.
Yes many times the easiest way to find the part is to get the heatsink/fan as one unit. Can be hard to find just the fan by itself. A heatsink is just a chunk of metal so it can't really break. Either the fan breaks or the mounting system holding the heatsink onto the cpu/gpu/chipset.