a couple places.. right off the top of my head - BUT, there are reports of so-so nicotine (as in bad smells / taste) that I can not say has been resolved or not.. look for the long thread "nicotine comparisons"
myfreedomsmokes offers 100% PG, 75/25, 50/50, 25/75 and 100% VG * normally I would recommend them, but as noted above, check to see if the issues have been fixed..
somewhere else I have seen that also offered 80/20 and 20/80 in addition to the blends that my freedomsmokes has, but can't remember where I saw them..
I mix my own and use nicotine from vaporshots, but there's is nic-select and what I am reading, nic-select is having the smell/taste issue also..
Kinda leaves you with wizard labs (not heard anything about them having issues) but they only offer 100% PG, 50/50, or 100% VG
and
Nude Nicotine (again, not heard anything about them) - they offer in 100% PG, 70/30, 50/50, 30/70 and 100% VG