I thought there was a feat or something that would let you gain access to a domain. This is for say a Paladin/Healer to gain access to a domain.
No feat does what you want
By far the easiest way to gain access to a domain is taking levels of cleric or a prestige class, as Thomas Jacobs notes. As KRyan notes, taking the feat Planar Touchstone (Planar Handbook 41-2) and picking the Catalogues of Enlightenment (166-7) really is as close as you're going to get, with the Catalogues' Higher Order Ability saying that
Once per day, you may cast a spell from the cleric domain you have chosen, as though you had prepared the spell normally. You must be of sufficient character level to cast the spell and have a Wisdom equal to 10 + the spell’s level.
However, this ability can only be used thrice ever before the creature must recharge the ability at the Catalogues, slaughtering his way through the Catalogues' guardian librarians.1
The following don't do what you want but might be the source of that nagging memory.
- The general feat Arcane Disciple (Complete Divine 79) grants a creature that casts arcane spells access a domain's spells as arcane spells. The general feat Divine Sorcery (Dragon #343 88-9) is similar but different.
- The epic feat Bonus Domain (Complete Divine 89) grants another domain, but using the feat's Benefit to prepare domain spells requires having domain spell slots.
The consumable magic item domain draught (Magic Item Compendium 156) (3,300 gp; 0 lbs.) allows the consumer to prepare the spells of the draught's domain in the consumer's domain slots. Specifically, the draught says that
if you normally prepare domain spells, you can add those from the draught’s domain to your choices until the effect expires [24 hours after being consumed].
In the same vein, the magic weapon Erythnul's Slaughter (Arms and Equipment Guide 106) (36,308 gp; 6 lbs.) grants a cleric the ability to prepare domain spells from the War domain in addition to his other domains, serving as a template for similar items.2
1 Okay, fine, to recharge the higher order ability you must win a philosophical debate against the guardians, but you'll probably wish you could slaughter your way through them instead.
2 Although I can't imagine many DMs allow this, the cleric class feature Deity, Domains, and Domain Spells (PH 32) can be emulated with a Use Magic Device skill check (DC 20 + character level). Ask the DM if a result of 37 allows preparation of the draught's or Slaughter's domain spells by a creature without the ability to cast or prepare cleric spells. Although that might sound ridiculous, bring the DM cookies and point out how Lidda activates the magic chalice by using turn undead attempts she doesn't have (PH 86).
There are no simple feats that do this: if you want a Domain you need to take a level in a Prestige Class. Also there is no such thing as the Divine Domain.
The simplest way to do this is via the Contemplative prestige class (Complete Divine, p. 30). This requires Knowledge (Religion) 13 though, so you'll get access to the domain of your choice at level 11.
If you have access to Eberron materials, you can go with the Sovereign Speaker (Faiths of Eberron, p. 32) which requires only Knowledge (Religion) 8 and a feat, so you can pick it up at level 6. But you are in a more limited scope with which domains you can pick with this though. And you have to worship the Sovereign Host, which might prove a problem.
Planar Touchstone is a feat in Planar Handbook that allows you to attune to certain notable planar locations. One of these is the Catalogues of Enlightenment, and the benefit for attuning to it is the granted domain power from any one Domain, as well as the ability to prepare one spell from that Domain per day, up to three times (and you can recharge by returning to the Catalogues). It doesn't have any requirements beyond being sixth level, though the actual attuning to the Catalogues is itself considered an 8th-level encounter.
And the Catalogues are only one of the many options from the feat. This thread has a good discussion of others.
Aside from that, prestige classes are your best bet; contemplative from Complete Divine lets you pick any one, but many, if not most, divine prestige classes give you particular domains.