Adtran may sell a fan kit.
Alternatively you can remove one of the fans and look for the actual manufacturer and model number on the label in the hub of the fan. Then check an electronics catalog such as Mouser, Digi-Key, Newark, etc and order the fans from them, or Google the number to find a distributor. If you go this route you will probably have to also get the connectors and crimp them to the leads of the fans as the raw part won't have connectors. The fans on most modern networking gear have a third wire used for a tachometer or sensor to monitor fan speed, so you need to ensure that you get the correct replacement.