to understand the situation better, do inmates have internet access? as odd as this sounds, have you worked with the prison system on this? seems they`d want to partner. are there specific groups catering to prisoner families already that you could partner with? also, there may be opportunities with religious groups inside and out of the prisons... this is not to discount any search optimization work you`ve determined drives traffic your way. but i have a hunch that this is one of those businesses that needs to BUILD. get some positive momentum going, create testimonials or "reference accounts" and parlay that into more business!
is this helpful?
How about posters? you would have to go directly to the prison administation, but if you could get the right permission, it seems that you may be able to put a poster in the visitation room. Something along the same lines would be information cards...or maybe a poster with information cards attached to the wall just outside of the visitation area.
Also, try to get your name out to local attorneys and bail agencies. Any link that gets a prisoner and/or their family knowlegable to your service is a foot in the door.