Always done my marine first aid stuff through St. John's Ambulance. Not sure what's required for SVOP, but it must be similar to what commercial marine officers are required to have?
It's not always obvious when internet searching which of their courses count for "marine first aid", but if you call them and tell them what your certificate is required to say, they'll lump you into the appropriate course. I've done it at a few different locations around the mainland as they expire, and the timing of various courses over the years worked out at different spots. Sometimes I was the only one in the class who's certificate said "marine" on it as requested.