Unseelie Fey, a +0 LA template grants DR based on your HD, but also extra damage against you from enemies using iron weaponry.
There are a bunch of templates that are "LA —" (i.e. Not For Players) that grant DR, amongst other things.
Here is a list of all the D&D 3.5e templates.
There are no items that grant a +1 enhancement bonus to any ability score. I have looked.
Ultimately, a +1 enhancement bonus to an ability score would welcome a lot of min-maxing in ability score point-buy (and, to a lesser extent, rolling or arrays). It would allow you to fairly trivially take an odd number for a score (saving points, especially since the following even number will have increased marginal cost for scores of 13 or higher), and get that last point with a cheap magic item. In short, such items allow characters to pretty easily boost their less-valuable ability scores, mitigating any weaknesses those lower scores would have caused (those scores important enough to get enhancement bonuses anyway, probably a primary score along with Constitution, don’t really change much since characters tend to try to get the largest enhancement bonus possible from them). It also lowers the effective “cost” of ability score requirements, for those who aren’t meeting them naturally (which I might buy would improve the game, but YMMV—Wizards, at least, didn’t seem to think so).
There are, however, the consumable manuals and tomes that, when read, grant a permanent inherent bonus to an ability score. Those are made with wish, and like wish itself, they can grant bonuses from +1 to +5.
But those inherent bonuses, as well as the ability score bumps every 4 levels, are far more valuable than the 1,000 gp of a hypothetical +1-enhancement bonus item, and thus are less “abusive” as alternatives to having the scores you need.
Best Answer
No effect directly, explicitly, and exactly duplicates an object instantaneously
The game doesn't want this to happen because it then expects the user of such an effect to go around duplicating +5 vorpal scimitars and stuff. Even were such an effect limited to mundane items, duplicating wealth like coins, gems, art objects, and trade goods quickly becomes an issue.
The nearest spell is the 0-level Sor/Wiz spell amanuensis [trans] (Spell Compendium 9), but even that spell remains problematic despite the fact that the "spell copies only nonmagical text, not illustrations or magical writings" because raw text exists that has value beyond the page on which its stored rather than its price as a work of art possessing intrinsic value.
Anyway, there are workarounds, but they require very careful consideration.
Rather than duplicating exactly, making the same thing repeatedly can be done using the 5th-level Sor/Wiz spell fabricate [trans] (PH 229). The magic item Murlynd's hat (Dragon #359 73-4) (72,000 gp; 0 lbs.), in addition to other effects, grants the wearer 3/day the ability to use an effect like the spell fabricate. A plot could be constructed around the painter being beholden to a fellow who occasionally lends the painter such a hat, with the lender seeking revenge after the PCs disrupt his income.
Alternatively, a rejkar (Monster Manual III 140) can use at will as a spell-like fabricate as a level 5 caster.
It can fabricate at will, and it's a genius. Don't judge.
According to the Planar Handbook a caster could substitute the rejkar for another creature on his list of creatures available for the spell summon monster V [conj] (PH 286, 287), but as the rejkar has only 5 Hit Dice it can also be bound using the 5th-level Sor/Wiz spell lesser planar binding [conj] (PH 261-2). Note that a rejkar normally has the Craft skills armorsmithing, carpentry, and weaponsmithing, but a generous DM may allow that to be changed as a house rule either by a skilled caster (e.g. a Knowledge (the planes) skill check (DC 25)) or a caster with an appropriate feat (e.g. Fiendish Summoning Specialist (PlH 39)). Even if it lacks the skill ranks, an untrained rejkar can take 10 on a Craft skill to hit DC 15. (A typical rejkar has an Intelligence score of 20. I'm not even kidding.)
Alternatively, a rejkar could be using the painter instead of vice versa. It's a crafty devil ram with abilities that make it, with some modification, an interesting low-level boss monster. (It's also easier for the DM to justify non-standard skills for a custom rejkar.)
As a spell-like ability fabricate has no material components, making it even more powerful and even more liable to break a campaign's economy casually than typical uses of the spell. Be careful.
The 6th-level corrupt spell consume likeness [necro] (Book of Vile Darkness 89) lends itself to a great many plots. Depending on the DM's interpretation of the spell's duplication of the consumed creature's equipment, the painter may not have even created the painting himself, instead having killed the original artist and cast the spell consume likeness on the original artist's corpse which had on its person at the time the painting that is now so popular and valuable. When the painter needs to sell another, he secretly assumes the likeness of the dead original painter, drops the painting, then assumes his own form once more, painting remaining intact.
This sets an incredibly dangerous precedent, but a manageable one if the spell's kept sufficiently obscure. In addition, the spell's also evil (presumably because cannibalism), and its effect is fragile. (The spell's permanent duration means an effect like dispel magic et al. ends the ersatz painter's consume likeness effect.)
To commit such a plot, a level 2 NPC could've purchased a single scroll of consume likeness (6th-level spell at caster level 11) (1,650 gp; 0 lbs.), committed the murder, and after deciphering the scroll gotten only marginally lucky on the caster level check (DC 12) to cast the spell. Thereafter, he'd likely avoid any possibility of exposure to an effect like the spell dispel magic, maybe even going so far as becoming mysterious recluse (or just buying a ring of counterspells (DMG 230) (4,000 gp; 0 lbs.) or adding his magic weapon the magic weapon special ability spellblade (PG 120) (6,000 gp; 0 lbs.)).
That said, any effect allowing duplicating items of value will be difficult to reign in once introduced. Weigh the ramifications carefully, and be sure to have a backup plan if things start going south, even if that plan is something as dumb as Asmodeus eradicating all the rejkars.