How do I become a correspondent to a child?

I see testimonials from others who have chosen to be correspondence sponsors to children who do not receive letters, but I cannot find a link to be able to sign up to do this. I would love to brighten a child's day with my letters, but do not have the financial ability to be able to monetarily sponsor another child (I am already a Compassion sponsor for a child). Please help me out- I love to write and would love to have another child or two to pour God's love into!