I thought I would share this place I found online. I haven't ordered yet one, so, I can't actually vouch for it, but, I am going to order one.
If you suscribe to get their newsletter, they send you a 10% off code for your first order.
My filter can't be bought in a regular HD, or Lowes, so, I have to pay for it at the heating place, which is 35 bucks. Here it is 26, plus, 10% off.