Where can I find all available co-crystallized protein-protein complexes? Is there any database of all these complexes where I can download the PDB files separately?
I am not sure you will find all protein-protein complexes, but at least some, I hope:
See also this post:
A: Query regarding RCSBPDB
And this one:
Finding Structures Of Protein Complexes!
See the following post for human protein complexes:
Where do I find flat files for human Protein complexes ?
For all crystal structures, your first port of call should be wwPDB and its partners. For example a search at PDBe for protein/protein complex gives >20000 results.