Where can I get RCX 1.0 software?
The EV3 software is not backwards compatible with RCX 1.0. 

You will need to install the software from the CD-ROM that would have come with your set. If you do decide to purchase one online, you can do so from a 3rd party marketplace such as Bricklink. 

Be sure to install the correct version of the CD-ROM (RCX 1.0) and check to see if your operating system is compatible with the software.