From the Advanced Race Guide, aasimars can use the Angelic Flesh feat to bypass cold iron damage reduction with their unarmed strikes and natural attacks.
From the Advanced Class Guide, a divine caster who worships a neutral deity can bypass DR/cold iron with Improved Weapon of the Chosen feat. The non-improved feat allows your attacks to become magical and hit incorporeal creatures.
The Frost Fist Amulet (10.000 gp) is a cheaper alternative to the Amulet of Mighty Fists +3 (36.000 gp). They can bypass DR/cold iron and gain the Frost weapon property (+1d6 cold damage).
This is an alchemical item, not a magical item, but the Weapon Blanch (Cold Iron) can coat any weapon to bypass DR/cold iron on the first successful hit. Let me add that you must put the weapon on a fire for a full round, so unless you have fire resistance (tiefling or ifrit, etc) you might take some damage doing so.
Catfolk can use Claw Blades made of any special material (or enchanted) together with their claws. The weapon has no damage listed on it's weapon entry, meaning that it doesn't replace their natural claw damage by the weapon's damage.
Ironbeard is a 1st level spell that make it grow a beard made of iron on your face, which you can use to attack as armor spikes that can bypass DR/cold iron.
Heart of Metal enchants a bunch of weapons (one/level) with the ability to ignore damage reduction from silver, cold iron or adamantine.
Versatile Weapon will enchant a single weapon so it can bypass damage reduction from one of the following: bludgeoning, cold iron, piercing, silver, or slashing.
The list now is pretty vast, but i will try to list those that are obtained at the earliest levels as possible, specially if you intend to multiclass to obtain their abilities.
At 1st level, an Occultist with a Transmutation Implement can pick the Philosopher's Touch focus power and spend 1 point from his mental focus to allow any weapon touched to be treated as silver or cold iron for 1 minute/level.
An Oracle of the Outer Rifts mystery can pick the Rift Weapons revelation, which allows you to once per day to make one weapon touched to bypass DR/cold iron for 1 minute/level.
A Silver Balladeer Bard archetype at 2nd level gains the ability to treat his silver weapons as if they already were made of cold iron for the purposes of bypassing damage reduction. Combined with Eldritch Claws, your natural weapons are considered made of silver and cold iron.
A Vigilante can pick the Expose Weakness talent at 2nd level, which allows him to use the Dirty Trick combat maneuver to reduce the target's damage reduction (or hardness) by 10 points.
A Kineticist could combine both the effects of Kinetic Fist, which allows you to add 1/3 of your blast damage to your natural/unarmed attacks, and Rare-Metal Infusion to turn his blast damage on the proper metal type for the purposes of bypassing damage reduction.
Monks can bypass DR/cold iron at 7th level with their Ki Strikes. Similarly, a Brawler gains this same ability at 9th level.
Barbarians with the Penetrating Bite rage power will allow his bite attacks to be treated as silver and cold iron starting at 7th level.
The Lantern Bearer prestige class can treat all metal weapons he wields as cold iron starting at 2nd level, which makes this available as soon as 7th level. There are ways to turn your unarmed or natural weapons into metal, like the Metal Domain, or wearing Gauntlets, which reduces your damage, but your unarmed strikes with your fists are now a metal weapon.
The Horizon Walker prestige class can align himself with a plane where the native creatures have DR/cold iron and his weapons will be able to bypass their reduction. This can be obtained as soon as 7th level.
The Nature Warden prestige class gains the Ironpaw ability at 8th level, which allows your natural weapons to bypass DR/cold iron. This takes a while though, as the requirements ask for at least 4-5 levels on other classes (easiest being ranger 4/druid 1).