Art Appraisal Letter Required

I'm seeking a professional art appraiser to provide an appraisal letter for an oil painting by Guerrero from East LA, painted in the 1960s. This letter is needed to substantiate the painting's value for a museum donation and must meet IRS requirements. Painting is about 40 x 28 inches and is of historical event: "Tree of Sad Night" with La Malinche consoling Cortez after a defeat to the Aztecs.

Requirements:
- Detailed appraisal letter
- Accurate valuation for IRS compliance
- No prior documentation available

Ideal skills and experience:
- Certified art appraiser
- Experience with IRS valuation requirements
- Strong knowledge of 1960s East LA art