Good writings on the body, including the cakras, can be found in
- Bouillier, Vé. & Tarabout, G. (Eds.) Images
du corps dans le monde
hindou CNRS éditions, 2002
- Michaels, A. & Wulf, C. (Eds.) The Body
in India: Ritual, Transgression, Performativity. Akademie Verlag, 2009,
18
On one of the earliest Sanskrit descriptions of cakras, see
- Heilijgers-Seelen, D. The System of Five Cakras
in Kubjikāmatatantra 14--16. Egbert Forsten, 1994
The tantra specialists will offer more precision, but the Kubjikamata and similar-period texts (Mālinīvijayottara) are among the earliest mentions of the cakra system as we know it. The dates are as Matthew said, end of the first millennium CE.
Yoga-Patanjali mentions the nābhi cakra in his Pātañjalayogaśāstra (SARIT), but to me this seems more related to medical models of the body, where sirās and dhamanīs converge on the nābhi, than to much later multi-cakra models of prāṇa and visualization that are developed in tantra.
Best,
Dominik