You'll want to determine what kind of a rock it is. The biggest concern will be that it alters the water chemistry--though with a high pH, that's not a huge concern. If you can't identify the rock, soak it in a bucket with water from your tank for 2 weeks, and then test the water. Compare the results--if there's no change in the hardness and KH, it's likely safe (inert). Check that the water hasn't clouded--some rocks will cloud the water, though they don't seem to change other readily testable parameters. Otherwise, rinse it and go!
No place that I've seen offers FW LR--most likely because the biggest reason to purchase cultured SW rock is not for the bacteria, but for the variety of organisms. Since there are not comparable FW organisms, there's no real reason to cultivate the rocks. You can use porous rock in FW, and it will become biologically active with the bacteria. Several types of plants will happily grow on these rocks--java fern and anubias sp most notably.
