The only way I can resolve it is that while we only see the goings on in Galactica (and sometimes Colonial One), there are numerous other ships in the fleet that harbor much less love for the military and for Roslyn.

We saw a while back that Zarek and his followers could mount substantial opposition to Roslyn. So it seems plausible that Balthar might have supporters on some other ships possibly with the facilities to print and distribute the book.

I'd assume a lot of folks also harbor a lot of resentment and blame for the military which retreated in the face of the Cylon arrival. And despite eventually rescuing the New Capricans, they were left behind to rot for a while before getting saved.

But I agree that it still seems like it would be tough for Balthar to garner that much support so quickly.