Need to differentiate between softsynths and softsamplers. Most acoustic instruments are softsamplers. The best out there are still PMI (as you pointed out) and sampletekk. They have been producing piano samples since ancient times and therefore more experience in it. NI, Steinberg etc are the upcoming potential - the late starters. CPU usage may be more taxing since they are individual VSTis as compared to the others that run on a separate softsampler. No problem if running only one VSTi piano, but when you start to have several plug-ins, you run into CPU and memory problems.
Good piano samples does not just depend on the number of samples/note but also the way the piano is recorded and the post-production programming. That's why people like Michel Post of PMI and Worra of Sampletekk are great in producing great sounding pianos - they specialises mainly in piano sampling.
There are others like Artvista which made a big impact when they first came out with the Malmsjo Grand - the darker version. They certainly rocked the piano sampling industry. That was a number of years ago. Now we get even more sample/note and Artvista slowly faded away to PMI and Sampletekk, the latter also sampled a Malmsjo White Grand - this time the brighter version for jazz and pop. It will be interesting to see where piano sampling heads.
As for where to purchase, the best way is online. I've asked many places who told me that to bring in samples are not worth it since the market is very small in Singapore. If they bring in samples, they cannot compete with the online stores which sell them at relatively low price. But you need to know that PMI, sampletekk etc need Gigastudio, Kontakt or Halion to run; unlike Ivory, NI's AP and the Grand 2 which are VSTis and run as plug-ins.
