The Apache Group web site (http://www.apache.org) usually offers the Apache server in source code form.
SCO's Web Site (http://www.sco.com/skunkware/)
SCO offers the software as a pre-built package tailored for SCO Open Server 5 and UnixWare systems, so it's usually easier to get it from them. However, their pre-built versions may not offer all of the modules you may want.