If that's a concern, you might want to consider software RAID. While it might be imposible in the future to find a "compatible" controller, it should always be possible to install an old version of your RAID software.

That said, RAID does not prevent you from having to create backups. It is only there so that you can need them less often due to disk failures. You should always count on needing a separate backup, in which case, the failure of a RAID controller (combined with the inability to find a compatible replacement) is simply one of the fewer instances where you need to restore from backup.