Level 1. There are several ways of doing this:
Some few player-character races have or can purchase via racial feats some small amount of damage reduction. The variant trait tables for Aasimar and Tieflings also have a small chance of granting alignment based DR.
haunted fey aspect is a level 0 spell that grants DR. You can gain access to this spell as a fighter via race, as above, or via the 'Magical Talent' magic trait. Certain other traits also grant access to sources of DR, and there may be a trait somewhere that grants DR directly, but there is certainly not one in the sources you list, and I don't know of any.
Many spells, like Iron Body, grant DR. Magic items that replicate those spells on an at-will or continual basis are therefore sources of DR. Some specific magic items, like the Hero's Hauberk, also grant DR. While effective, most of these are too expensive to purchase at level one, even with the 1800 gp you can potentially start with.
Some mundane equipment, most notably Adamantine armor, grants DR.
Race is probably the best option if you want this to be a core feature of your character at as low a level as possible. Gargoyles are probably the best choices for this, especially if you can apply the gemstone gargoyle template.