The liner paints from Reaper are just concentrated dark paints that are a little more concentrated than the standard paints (halfway between inks and paint from what I've read) so that you get good colour at an easy brushing consistency. A good-quality standard paint thinned with medium, or maybe just water, could stand in their stead; a home blend of brown ink and paint could be mixed if necessary.
Not sure where you're based but a fluid acrylic paint from one of the artists'-acrylic makers would be worth trying as an alternative. These are widely available online if you don't have any bricks-and-mortar retailers of art materials nearby.
Einion
