Maintenance
is the best source for liquid nicotine. I have never heard, nor do I think exists, of nicotine in a solid form outside of tobacco. 100mg is for people who smoke more than a pack a day before they started vaping. 60mg is suitable for the average vaper. 48mg and below is best for those who barely smoked or those who only smoked socially. I recommend 100% PG base because, between PG and VG, PG is the more expensive of the two.
You can get VG at Walmart in the pharmacy/First Aid section for around $3 for darn near a year supply. It's USP grade which is what you want and it is labeled as a "skin softener" under the name on the label, simply "Glycerin". It will also confirm on said label that is is VG and that it is USP grade.