Methods are for PADI only. For other agencies you will have to contact directly.
One method is to contact your training agency by phone if you lost your certification card and replace it directly with them. Here is the number to PADI: Phone: 800 729 7234 (US and Canada), +1 949 858 7234.
Another way to replace a lost scuba cert card is to follow this link to use PADI’s online application. Here you can purchase the e-card, upgrade from Junior Diver and even select a cool artsy background for your new shiny card all online.
To upgrade your card from Junior to Standard certification level, please complete the PADI Diver Replacement Card Application online for your Junior certification level, and your card will print with the new/updated level.
If all else fails call your LDS(local dive shop).