I'd try your local Volvo dealer for service items like headlamp wiper blades. Volvo part number 274434. Last time I bought a pair they were pretty cheap.
Or FRF Volvo dealers in Swansea do mail order and give a huge discount if you mention you're from the Volvo owners Club forum - discount code C3.
https://www.frfvolvo.co.uk/
Brookhouse are a good parts source for older Volvos -
https://classicvolvoparts.co.uk/sear...&Submit=Search
Finally, Skandix (in Germany) - they normally sell original Volvo parts, or aftermarket. They seem to only have aftermarket at present.
https://www.skandix.de/en/spare-part...aning/1040365/
As John said, the headlamp wash system operates automatically every time you wash the windscreen, so don't leave it too long without a headlamp wiper blade as you'll scratch the headlamp.
Good luck with your new car - the early 80s facelift 240s are quite rare now and have some nice features not present on later cars.
Regards
Lucien