. Can PCP's be filled from a small air compressor or the air tanks used to inflate tires?
No. The pressure required is far beyond anything that a small pump or compressor will put out - about 200BAR or 3000psi.
As noted by the previous poster....a special pump is needed. A very special three stage pump. The pumps run about $250+/-.
The Discovery is a little easier and operates at a lower pressure, so does the pump.
I started with a pump and, when I could afford more, I bought an 80 cu.ft. 3000 psi scuba tank. PCPs are great fun and there is no comparison to their power, especially the big bores 9mm/.45/.50.
Pete