The fact that the motor is trying to turn over indicates the starter motor is probably ok, If this is so you need a new battery. You don't have to buy a saab battery, any reputable brand will do. You need a 70 Ah battery then can produce 600CCA and which preferably has 11 plates.
Take these specs to the shop and you will be fine. Make sure the terminals on the battery are the same way around as your exisitng battery.