You don't need it- it isn't going to add any benefit of being in your loop unless you are running aluminum in your loop.
Distilled and some biocide works fine. I picked some up at Walmart for like $3 in the fish food area, but a lot of people get stuff online like PTnuke from Petra's Tech Shop, for example. Most sites that sell watercooling gear sell biocides. You should at the very least run biocide in your water, kill coil is optional.