What is the ultimate source of ADP/ATP in humans?
Phosphorus is a very common nutrient, found in high levels in proteins, which are in such foods as milk and milk products, meat, beans, lentils, nuts, and grains, especially whole grains. Phosphorus is found in smaller amounts in vegetables and fruit, as well. Adenosine can be found in many of the same foods, both as free ATP/ADP/AMP, and as part of the DNA found in all animal and plant-based foods (are there any other kinds?).

Adenine (the nucleotide base before the addition of ribose creates adenosine) is synthesized via the purine synthesis pathway, whose ultimate product is IMP, or inosine monophosphate. IMP is then converted to adenylosuccinate, then to AMP, which is then phosphorylated to ATP. It should be noted that, as is the case in most biosynthetic pathways, this one requires ATP to power it along, meaning it takes ATP to make ATP.