They offer them for $27 direct from there store.
SOLAR POWERED ROOF VENT (Round Base) - SolarSales
I know nothing about the company but happened to go to there website to see if they had anything else useful.
You can't just use a lithium battery. Do they not have a battery at all in them now? There would be some circuitry you would need to add a battery to them. There are some that have a battery already, here is one example but it does yellow in the sun and the motors are pretty weak.
Wel-Bilt Solar Vent | Ventilation| Northern Tool + Equipment
After I got them on eBay (made an offer for $25 plus shipping) I found them in other places as well, by the time you add shipping, it all ended up about the same. I was pretty excited to have found the things on eBay, so I went ahead and got 'em. To reiterate, these are EXACTLY what has been on the boat since I think maybe 2006. With this in mind, I think they are a good choice, as fitment will not be an issue at all.
As part of the marinizing of them, I will take them apart and paint them white with Krylon or something like a previous owner did with the previous set. They currently have no battery, but a poster in the Solar vent repair thread suggested NiCad or NimH instead of lithium, and that a diode to prevent the batteries from discharging into the cell would be advised. Now I plan to get a set of 4 AAA batts for each fan, install them serial/parallel so that they end up at 2.4V, but double the maH capacity from a single pair.
Cheapskateness sometimes isn't as cheap as it first appears, but for me it is also the sport that makes it fun.