The is not a product; it is a relationship between a legacy device and a shrinking pool of compatible storage. The best card for your NSDD-W61 is an 8GB or 16GB SDHC card (Class 4 or Class 10 U1), formatted to FAT32 with 4K clusters.
The NSDD-W61 (and associated technologies under this designation) belongs to a class of "Self-Destructing" or "Instant-Sanitization" storage media. Unlike standard consumer SD cards, which merely mark data as deleted (leaving the actual magnetic or electrical charges intact until overwritten), the NSDD-W61 is engineered to render stored data mathematically and physically irrecoverable in a matter of milliseconds. nsdd-w61 sd card