Network Solutions and some others sell 'wildcard' certificates, which I use. You request them from one IIS server for a wildcard of your domain, say, *.mydomain.com and then it will work on any server answering to that domain name. I think the cost was a few hundred dollars for four years, which actually work out pretty well when you consider the flexibility it gives you. I have exported the certs and used them in routers, mail appliances, Linux-based VM's, you name it, and gotten a lot of value for the money spent.
