Have you tried a PDB advanced search?

Try opening the Molecular Function Tree and searching for RNA polymerase. You can then further refine the search by clicking on Bacteria in the results. Or you could open the NCBI organism tree to add specific groups of organisms.

I found 138 bacterial DNA-directed RNA polymerase structures. I don't know how many of those are from Gram-positives; there were none amongst the Firmicutes.