Also with some acro you may not see any significant growth from the piece until it encrusts on the frag plug or rock.
Here is a great place for acrylic rod. Get the extruded as it is cheaper. They have various diameters and it is sold by the foot.
Clear Extruded Acrylic Rod | U.S. Plastic Corp.
I don't like plugs or rods for frags so if the frag/rubble rock is too heavy I use combination of aquamend epoxy putty sold at Lowes and HD along with super glue gel.
Great link! I think I may have to place an order with those folks. Need to get some 3/8 for aquascaping too.