EDIT. There are very many published proofs. For example, Colin de VerdiereVerdière (Forum Math., 1 (1989) 395-402), or Charles R. Collins and Kenneth Stephenson, A circle packing algorithm, Computational Geometry 25 (2003) 233–256.
Thurston's original proof is elementary: he gives an algorithm for finding the (essentially unique) circle packing from a triangulation. He never published this properly; his "Geometry and Topology of 3-manifolds" exists as a preprint, and the proof there are sketchy. Then Colin de VerdiereVerdière (Forum Math., 1 (1989) 395-402) proved accurately convergence of Thurston's algorithm. Another proof of convergence and implementation on computer is in the paper of Collins and Stephenson mentioned above. Then there were many generalizations and versions of this theorem and algorithm, too many to list all of them here.