I had the same problem in finding that aluminum washer at local part stores. I ended up buying washers and some other parts from eEuroparts which included the free shipping. However, just ordering the washers ($ 0.30) makes it quite expensive with standard ground shipping of $11.
Local parts stores suggested to use a copper washer. I don't know what the reason is for Saab to spec an aluminum washer but I didn't want to take a chance.
eEuroparts - Aluminum Washer - # 977751
I re-used the old washer for the first few changes and never noticed any leaking. eEuroparts shows BMW and Volvo also uses this same washer for (some of) their vehicles so you can also check with those dealers if there isn't a Saab dealer close to you.