I dunno about wiper blades from scrapyards. Chances are, they'll be knackered. How about doing an Ebay search (and save it) using a few part numbers.
Have a look at
http://ecat-online.bosch.de/toc/
and have your chassis number handy. This'll tell you which cars have the same headlamp wiper blades as yours.
#---------#
Up to Ch nr. 120999, Ford Sierra MkII and Volvo 740/760 headlamp wiper blades will apparently fit a 300.
Bosch 3 398 113 084
equivalent to
FORD 6151228
FORD 87BG 13K089 AA
VOLVO 1325545
#---------#
From Ch. nr. 121000 onwards it's a bit more difficult - they list Fiat Ducatos, '83 - '85 Honda Accords, Opel Mantas, Peugeot 604's, and Renault 5's (were they ever fitted with headlamp wipers?)
Bosch 3 398 113 041
equivalent to, well, lots:
CITROËN 6430 79
DAF 278216
FIAT 5958240
HONDA 38547-SA5-622 HE
IVECO 5958240
LANCIA 5958240
OPEL 09283851
OPEL 1271421
PEUGEOT 6430 79
RENAULT 77 01 030 949
VOLVO 278216
VOLVO 32782161
Try an Ebay search with some of those part numbers, checking the box marked "Include title and description"
Cheers
Tom