Connecting Network Extender to Mac Airport Express

I have the Mac Airport Express router because of its ability to wireless stream my music to my speakers.  Unfortunately, the Airport Express only has 1 ethernet port to connect to a modem.  My modem only has 1 ethernet port.  I have nowhere to connect the Network Extender.  I went to my internet service provider and they stated they do not have a modem available with additional ethernet ports.  Can anyone help me with a solution to this problem?  It's probably something simple; I'm not exactly tech savvy.


I'm trying to avoid buying a new router and just using the Airport has a client on that network.


Thanks for any help.