You will need to get some PG. To make 24, just equal parts of your nicotine and pg will get you 75%pg 25%VG 24mg liquid. You can also use VG to dilute it, or a mixture of pg and vg to get a different ratio, but as long as you cut it in half it will be 24mg.
VG is easier to get locally, but your mix would probably turn out too thick if you use vg to dilute. Probably would be best to find a place to order a small bottle of pg with free shipping.