Both are available, but the NMOS are a lot more easy to get, as looking for PMOS taught me. Basically NMOS are smaller than PMOS given the same specs.The main disadvantage of this type of MOSFET is the poor on-state performance: it uses holes as charge carriers, which have a much lower mobility than electrons. As resistivity is directly related to mobility, a given PMOS will have a RDSon three times higher than a N-MOSFET with the same dimensions
In the old days the p-channel was more common. The process for it was developed first but as modern techniques were developed we were able to take advantage of the better physical properties of a n-channel device.Actually you have it backwards, NMOS is by far the most common of the two, but both exist.