I thought you were just looking for the Fire domain, it has the spells there.
What were you looking for?
EDIT: Oh, it seems like you already knew what the feat did so I was just pointing you to the Fire domain. I do not know if the feat is available anywhere online legally.
Domain Access (CC, p 52): Lose one 2nd level spell known, and one spell known each time you gain a new spell level. Gain access to a domain, including domain power, and can cast a domain spell of each level once per day.
That should be the correct source since the class feature is called 'Divine Sorcerer' and does exactly what the OP described (though it's a pity the DM decided it had to be the Fire domain...).There's also an alternate class feature from Complete Champion for Sorcerers in 3.5 that grants divine domain access
There's Arcane Disciple in 3.5 which has the same effect. Here's a thread where it's adaption to Pathfinder is discussed.
And then there's the (also 3.5) Fire Bloodline feat from Dragon #311/ Dungeon Compendium Vol. 1, which is a sorcerer feat granting fire spells, but not exactly the domain.
There's also an alternate class feature from Complete Champion for Sorcerers in 3.5 that grants divine domain access:
I've found nothing for Pathfinder though. Perhaps it's a homemade feat?
That should be the correct source since the class feature is called 'Divine Sorcerer' and does exactly what the OP described (though it's a pity the DM decided it had to be the Fire domain...).