You can support the work of the Global Fund by making a contribution to the U.S. Fund for the Global Fund. The U.S. Fund for the Global Fund is a charitable organization, therefore U.S. taxpayers can deduct their contributions as allowed by law.
We also encourage you to visit Product(RED), which has several donors that match contributions (though these do not come to us, they do help support the Global Fund!).
Amazon users can support the U.S. Fund for the Global Fund by using Amazon Smile, which offers a small percentage of purchases as a contribution (and no additional cost to you!).
Employees of Apple, Chevron, and HP can designate the U.S. Fund for the Global Fund through their respective employee giving programs.