There are several ways to get an extra domain if you are allowed the source by the DM
Complete Divine has 7 classes in a nice list on page 20 at the beginning of the chapter on prestige classes, I'll point out the contemplative because at you get an additional domain and power at first level and 6th level.
From Faerun
ANY
Divine disciple 4th level of a 5 level class grants an extra domain and the power from that domain (Players guide to faerun pg 51)
Gond
Techsmith 3rd level of 10 level class grants an extra domain and the power from that domain (Faiths and Pantheons page 207)
Of course there may well be ones I have missed
Complete Divine has 7 classes in a nice list on page 20 at the beginning of the chapter on prestige classes, I'll point out the contemplative because at you get an additional domain and power at first level and 6th level.
From Faerun
ANY
Divine disciple 4th level of a 5 level class grants an extra domain and the power from that domain (Players guide to faerun pg 51)
Gond
Techsmith 3rd level of 10 level class grants an extra domain and the power from that domain (Faiths and Pantheons page 207)
Of course there may well be ones I have missed
