There are numerous ways to get additional d6’s worth of sneak attack, but almost all of them require that you have 1d6 (or more) sneak attack before you can take them. How does one get +1d6 sneak attack damage from non-level sources when one has no sneak attack?
There is at least one way—Martial Stance (assassin’s stance) from Tome of Battle, which actually gives +2d6—but that isn’t available until 12th or (if the DM is generous) maybe 9th, and requires another feat besides.
- Any Wizards of the Coast published 3.5e material, as well as any 3.5e content from Dragon and Dungeon magazines, is legitimate.
- While stuff earlier than Martial Stance’s 12th is preferred, any content available pre-epic is acceptable.
- Epic content is not allowed, even if you somehow cheese into it prior to epic levels.
- Any kind of polymorph or shape changing is not allowed.
- The sneak attack must be available on a continuous, permanent basis. A magic item is acceptable, as is a 24-hour daily effect (e.g. spell), but not anything less than that.
- Persistent Spell, and any other means of turning a less-than-24-hour-duration spell into a 24-hours duration spell, are not allowed.
- LA counts as “levels” for this purpose, as do RHD. No ECL can be consumed by a valid answer.
- Bloodlines are not allowed.
- Any form of “undoing” levels (e.g. level loss from energy drain or resurrection, curing lycanthropy, various rituals) is not allowed.
- No assumptions about which classes are taken may be made, though you may leave achieving the necessary BAB/saves/skill ranks as an exercise for the reader so long as they’re non-epic values.
Really, what I want is feats and/or magic items that just say “you gain +1d6 (or more) sneak attack,” not some kind of shenanigan. Assassin’s stance proves it exists, and I’m fairly confident there’s some more out there (some named magic weapons, IIRC?). If you’re inclined to suggest a shenanigan, you may be well served running it by me in a comment before you spend your time—I have made every effort to provide a thorough list of the things I’m against, but if an answer manages to prove me wrong, I’m still not going to be happy with the shenanigan.