Looking for a Dropbox-style syncing solution for off-site employees

We have several locations all connecting to our primary site via Citrix. We need a solution that allows us to sync a particular set of folders to computers off-site, over the internet. We need this in case of an internet or power outage at our main site. The off-site file cache will only be used in the event of an emergency, and hopefully when services at the main office return, any changes made to the files would then be synced back to the main office.

We have looked into VPNs and basic syncing software such as DirSync Pro and built-in Windows offline files, but that is not what we are looking for.  We thought Dropbox would be a good solution, but when we contacted sales they told us their service was a "server replacement" and would not support our setup.

I'm looking for a service that will take care of file synchronization for us.