The optipoints have been no longer manufactured or supported for many many years. There is no official download point for the firmware anymore. If your department decided to keep the phones on those slightly older firmwares, they should alas have also decided to keep the firmware itself.
If you want the last released version, and it's not on the FTP server people use here, I would probably have it. It's no great upgrade step for you, 1 or 2 minor versions from what you have. It's still about 8/9 years old though. I don't know of a case where a phone on HFA V5 R6.7.0 had to be downgraded, it will work fine for you.
If your phones are actually defective, then I don't know if the firmware is the right solution for you, you might need netboot to make a low level flash on the phone. If (by chance) you have a problem with the display, firmware will not fix that.