I'll give you a minute to laugh at the title.
Seaman is one of the strangest and most interesting titles the Dreamcast ever had. It's a take on the "virtual pet" genre, only the pet you're taking care of is a talking fish with the face of a man. Creepy? Absolutely.
When I found this game hiding in the audio CD bin at Goodwill, my initial reaction was to leave it behind. After all, it requires the Dreamcast microphone, which generally goes for about $25 by itself. A copy of Seaman with the mic generally runs from $30-40. It'd still be a deal, but I'm a cheap-ass collector.
When I found it, still in the CD bin on a color tag sale day ($2!), I gave in and took it home.
So began the quest to find a cheap microphone. Ebay and Amazon yielded the expected prices, and nothing popped up on Craigslist. Before I got the chance to check into any local game stores, I remembered that Seaman wasn't the only game that released with the Dreamcast microphone.
Alien Front Online - it's cheap and fun.
The above copy of Alien Front Online, complete and boxed, came from a seller on Amazon.com. $7.98 shipped.
Apparently, even though it's a great game and includes a hard-to-find peripheral for a system that's slowly sinking into obscurity, these are dirt cheap. As of this posting, a factory sealed copy of this game is only $13 on Amazon. Personally, I recommend striking while the iron is hot if you're a Dreamcast collector in need of a mic.