I don't believe there is a universal cross reference available on-line that will give you all the comparable parts from all manufacturers.
There is a company that specializes in semiconductor replacement parts, and they do have a web site. They also offer datasheets on-line for any part they have that matches your search.
I tried your example and got NTE621 is equivalent to PRLL4001. You can look the datasheet up yourself on their site.
Many distributors carry the NTE parts. NTE now also owns ECG which used to be a competitor run by Phillips for replacement semiconductors.
The web site is:
h**p://www.nteinc.com (replace * with t)