Many universities which have medical schools training physicians nowadays also have physician assistant tracing programs. To be competitive for acceptance into one of these programs you should already have an undergraduate degree in one of the sciences like biology or chemistry. You can also find out more info by going to the web site for the American Academy of Physician Assistants.
As far as I know there are no post-grad programs for hematology. There are for some specialties such as orthopedics and I'm sure more and more training programs for specialty and sub-specialty PA's will become available in the future. But for now PAs who wish to work in a specialty seek positions in private or group practices or in hospital settings. For example, I live in Atlanta and had an occasion to be seen as a patient at the Emory Unversity Spine center where I was to see a neurosurgeon. But the first person I saw was the neurosurgeon's PA, a 2014 graduate of the Emory Medical School's PA program.