I have tried going to the RCSB website and quite frankly it was useless since when I looked up receptors like say the metabotropic glutamate receptor 2 (mGluR2) I got no helpful results.
As pointed out in the comments, if you look for PDB files and you don't find them in the PDB, there is no solved structure. However, you can look in databases that contain models of proteins. Especially for the receptors the coverage has been increasing in recent years and so you may be lucky.
A good starting place to look for protein information is UniProt, where you find links to the Protein Model Portal and SwissModel. They contain models based on the PDB structure for Metabotropic glutamate receptor 3.