Where should I look to find Ordinary Seamen positions available in the Merchant Marines?

I'm trying to help my boyfriend find Ordinary Seaman positions, but I haven't found much luck on finding Merchant Mariner Company's to begin with. He already has his MMD and other doents needed. He's looking for an Ordinary Seaman's position. I'm very much unfamiliar with the Merchant Marines, so I don't know if I'm even searching right. A point in the right direction would be greatly appreciated!