All of the below is my impression, I might be completely wrong, don't take my word as gospel and definitely check everything you might book yourself first!
Looks like the easiest place to fly in to is Toulouse. Easyjet fly there from Amsterdam and London, so it shouldn't be too hard to get reasonable flights. BA via Heathrow is another option. Bordeaux may be another option if you want to relive the memories of last year's win. Looks like it's an hour on the train from each.
Agen looks like quite a pretty little place. Population just over 30,000, with some nice old cathedrals and churches it seems. According to wiki the average weather for November is high 15c low 5C, 10 rainy days in the month.
Agen Rugby play at the Stade Armandie, capacity 14,000