Of course if you only have income from self-employment you won't have a W-2. In addition to employee elective deferral and voluntary employee contributions making you eligible for the credit, regular personal contributions to a traditional or Roth IRA qualify for the credit. However, since SEP contributions are employer contributions, they don't qualify (except for employee elective deferrals to a SARSEP established before 1997).
You might want to consider making a contribution to a traditional or Roth IRA if you otherwise qualify for this tax credit.