Where to find the form number needed to complete certifying the need for in-home care?

The VA uses their own form 21-2680:
http://www.vba.va.gov/pubs/forms/VBA-21-2680-ARE.pdf
Your county social services may have their own form but certainly they would accept the federal form as a first step.

I went to the Internet and typed in "In-Home Supportive Services" and many hits came up. Sounds like something used in California. You would need to check this out yourself depending on your situation.