On other Stack sites it is quite common for purchasing assistance and shopping ideas to be off topic. The reason being these types of questions become outdated very quickly as technology changes, a store goes out of business, or a product is no longer available.
When looking at this: Makerbot Smart extruder+ heat block and nozzle question - there is the basis for asking about Mk10/Mk11 printer heads, but they also ask about where to purchase them.
Are these types of questions on-topic for 3D Printing?