Hi, Jes,
My "foot bag" is one of the most valuable parts of my pack. In it, I have the following: a few compeed of different shapes (buy those in Spain at the start, there are US imitations, but I think they are inferior), some regular bandaids, a small tube of anti-bacterial cream, some gauze pads, small scissors, some cloth tape (called Omnifix in Spain, it's terrific), a tube of Voltaren (anti-inflammatory cream, again, to be bought in Spain), some moleskin, and most importantly, molefoam. This is a much thicker version of moleskin and I use it to "build up" the area around a blister so that the blister itself is protected from contact with the shoe. That was a trick my emergency medical responder son-in-law gave me once, and it has worked miracles on many blisters. I used to carry lambswool but realized I never used it.
I don't know if there are pre-made "blister kits" but I don't think it's too daunting a task -- mine fits easily in a small ziplock bag, so it doesn't have to be a huge undertaking.