The 5638 is still in production, for example:
http://www.mouser.com/index.cfm?&handler=data.listcategory&D=*2N5638*&terms=2N5638&Ntt=*2N5638*&Dk=1&Ns=SField&N=0&crc=true
However, there are a number of FETs from that same process including the PN4391.