Typical installations are for Report Services to be installed on a standalone server. These days that is usually a Virtual Machine on a cluster, so you get some high availability from the VM cluster. If you need to scale out (done more for spreading out load, rather than for high availability), you should be able to put a series of Reporting Services machines behind a load balancer. I have not done this, but ReportServer is basically a web server. The largest considerations will be configuring the DNS name so the individual Report Servers can recognize the requests properly.
I would go back to the users, and see if the uptime provided by your current environment for a standalone server will meet their needs, or if they really want to spend the money of setting up a load balancer, and a series of ReportServers. My guess is when shown the price tag, they will agree to the 99.9% uptime option, rather than the 99.999%.