I forgot about the B&C's carry out facility. It is definitley the best offie in the city centre, but I'm not sure what they have in the way of specific Oktoberfest beer.
The Lowenbrau Oktoberfestbier might still be available in your local ALDI. I'm not that fussed on it; it's an up-scaled (6.1% abv) version of the reg'lar Lowenbrau.