There aren't many new Standard ML libraries being written these days. Standard ML is almost dead as I explained previously in Is Standard ML dead?. To get libraries, here are a few places you could try:
Includes useful libraries for networking, JSON, XML, regex, command line parsing, etc.
Home to smackage, the package manager for SML, readline bindings, cURL bindings, gzip bindings, bz2 bindings, Cairo bindings, a CGI library, an extended Basis Library, etc.
There aren't many libraries, but the existing ones are worth looking into. On my wishlist is a comma-separated values (CSV) library for data analysis tasks, but there doesn't seem to be any other demand for it.
The documentation for the SML Basis Library can be found at https://smlfamily.github.io/Basis/index.html