Dear Shahnaz,
SEI is the authority for any Capability Model, be it CMM, CMMI, or PCMM. They are the founders/creators of these Models. However, they do not publish the results. It is solely at the discretion of people in the org to declare their status of assessment. Although most companies would announce their assessments and use it as a marketing tool, it is not mandatory, and there are some companies that have done it solely for internal improvement!
You can't find the full list of companies who are PCMM assessed on the Net. You would have to call up companies and know from the horse's mouth!
Cheers,
Kanishka