Have you tried the Twenty Mule Team laundry booster Muddy. It is said to be toxic to ants in extremely small quantities. I think the Terro stuff is just boric acid mixed with a sugary syrup. It also kills termites. I cannot say what difference in effectiveness there is between the boric acid formulation and the regular borax, but they look identical other than the Terro stuff is ground to a fine powder. You can get it at less than a dollar a pound at wallyworld.
It is not however safe around all plants, but neither is the Terro stuff. The USDA found it to be an effective herbicide for Creeping Charlie.