How to buy more terminal service licenses.

We run a windows 2000 network and have a couple of MAC users in our marketing dept.  They need to have access to a windows machine and I don't want to put additional pc's at their desks.  I have set up a Windows 2000 Server which allows 2 remote connections to it at any given time and that is working for right now, but I need 4 connections soon.  Is there a way I can do this?  And if it requires purchasing something do you know where I can get it?