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 However, they do not publish the results.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 on the discretion of people in the ORgn to declare their status of Assessment.
Although, most companies would announce their Assessments and use it as a Mktg tool, it is not mandatory and there are some companies who have done it solely for internal improvement!
You cant 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