Montblanc does not supply individual parts to private customers. Anyone wanting to replace a broken component, the clear connecting piece between the front part and the section for instance, is referred by the factory to its chargeable service, which means sending the whole pen in. For older models such as the Classic series of the 1980s, which is close in form to the 221, that is often uneconomic.
In the collecting world there is no spare-parts store in the proper sense. Specialist dealers and workshops such as Tom Westerich, Horst Schrage or Lutz Fiebig do keep large stocks, but they recover their parts from broken pens themselves. Prices for a single component are correspondingly higher than one would expect of a mass-produced spare.
In practice the cheapest route is therefore usually to buy a comparable pen from the same series, a defective one will do perfectly well, as a donor and to swap the part over yourself. With the widely available cartridge pens of the 1980s the supply is large enough that a donor can as a rule be had for less money than the single spare part costs.