I have a Whirlpool Gold under sink filtration system that takes three replacement cartridges - WHAB-6009, WHAB-6010 and WHAB-6011. These are filters A, B and C, respectively. I cannot find these filters on your site or any other filter site, for that matter. Are these available only from Lowe's, which is where I bought the system? Or is there another filter that is compatible with this system?