In short, doing an 'msfupdate' will get you the latest Metasploit-compatible exploits. The developers do a good job of including modules from 3rd parties, so it's safe to say that this will get you most of the public modules.
As other posters have indicated, Metasploit requires a specific format to utilize an exploit. Metasploit exploits are often (always?) posted on ExploitDB, but are almost always included in the framework itself as well, so loading an exploit from ExploitDB using the 'loadpath' command won't do much good.
Any exploit is theoretically Metasploit compatible, but will need to be ported. This is a decent place to start: https://community.rapid7.com/docs/DOC-1275
There are some beginnings of public 'exploit packs' for metasploit, check out Mubix's 'q' if you're interested: https://github.com/mubix/q - Though these modules are often incomplete or broken for various reasons, much like the 'unstable' branch of the metasploit-framework: https://github.com/rapid7/metasploit-framework/tree/unstable/unstable-modules