In the US there are several suppliers where you can buy unflavored pre-mixed at 18 mg - but you are paying a premium for that if you buy it in any quantity. You are buying mostly base liquid, relatively little nic, and shipping by weight is still shipping by weight.
Should you buy PG or VG with no nic locally it's easy enough to cut that 36 mg/ml - or any other concentration - down to whatever you need. You can find calculators to do that, and all sorts of other help, in the DIY forum. ~>
DIY E-Liquid