Where can I buy a windscreen-wiper motor linkage rod?

It all depends on what sort of car we're talking about. If it's a relatively new car, you might have to buy a brand-new part. Even then, there are options, because you can buy a genuine replacement part from a dealership, or shop around for a differently-branded part (that may well be the same part number, from the same manufacturer, but in different packaging). Just be careful if you're shopping online as plenty of people have been scammed by fake websites offering the parts they need.

If it's an older car, don't rule out the good old parts recycling yard (or wrecking yard as those of us of a certain age know them). Something like a wiper linkage is unlikely to be in high demand, so if the yard has your make and model in stock, there's a fair chance it might have the part you need. The only catch here is that some yards insist you remove the part yourself, so you might have to take your tools with you. Generally speaking, a second-hand part from a wrecking yard should be a fraction of the cost of a new part. But check it carefully, as there are often no guarantees with second-hand parts.