The problem is that the group 48 battery made by Johnson Controls is actually a group 48/91 battery and is therefore about 15mm shorter (175mm)
The same is true of the group 48 battery made by Exide. It's also shorter than a "real" group 48 battery.
I checked AC Delco (Delphi's) website, they only offer a Group 48/91 battery that is again 15mm too short.
The above three manufacturers probably represent about 95% of the batteries sold in the USA.... (incidentally, batteries in Europe are not sold by group size, but by DIN or ETN size, so if you've never heard of that before, that's why).
The rationale is probably that a group 91 battery fits in most places where a group 48 does, plus all those places where a group 91 battery fits, so by making a dual-application group 48/91 battery they are able to consolidate inventory and, well, if people who buy a car that sells 25,000 in a year have to file down their battery box to make it fit, well, too bad.
The only battery I know of which is supposedly (I'm going by what they say on their website--I haven't measured one) identical in height, width, and length to the stock Saab battery is made by Deka/East Penn, is a group 48 battery, and I know of only two places that sell it: Batteries Plus (as a "Werker" battery) and Carquest. Needless to say, Deka/East Penn isn't one of the larger battery manufacturers here in the USA.. But you can read about it here:
http://www.eastpenn-deka.com/assets/base/0379.pdf
The other option is to use a group 47 battery. It should be the same height as the stock Saab battery, 190mm[1]. But it has less capacity than than a group 48/91 battery, since it is not as long. The Saab battery tray has an additional hole for the hold-down to accomodate a shorter battery, which should make it fit the group 47 battery.
[1]I haven't measured, but a "short" group 47 battery would be the same phsyical size as a group 42 battery(175mm), and I've compared the capacity between the group 42 batteries made by Johnson Controls and the group 47 batteries made by Johnson Controls and the group 47 has more capacity, which indicates to me that it is indeed physically larger than the group 42.