Guanine was initially found in guano, that is bird and bat droppings, frequently mined for fertilizer because it is so rich in nitrogen. Guanine has five nitrogens per molecule, so it is a good vehicle for the excretion of waste nitrogen in a concentrated form. Uracil is found in the urine of all carnivorous mammals. Il means a substance related to, therefore uracil is a substance related to urine and acetic acid. It probably precipitates from urine when acetic acid is added.
Thymine comes from thymus gland.
Glandular tissue is rich in nucleic acids and it was also the source of adenine. Aden is a greek root meaning gland. I could not find out where cytosine got its name, but the prefix cyto means cell, so it probably was extracted from some type of cell. For Homo sapiens see Human Purine Metabolism or just use the pull down menu. This figure looks intimidating, but it is really not so complex as it seems at first.