From @Queuetue : I find boston rounds to be the very best bottles for mixing and temporary storage. I use them for my VG and PG as well as on the scale while I'm making a mix. With a top that's easy to fill, easy to cap and with minimal access to air, I keep a bunch on hand for unflavored base, or whatever project I may be up to for handy, quick access.
The tip is small enough to use with most top-fill RTAs or RDAs as well.
Mixing your own eliquid is actually pretty easy, and once you get started, it can save you a lot of money.
This video by New Amsterdam Vape lays out almost all of the basics in a really helpful way:
Here are some great resources to buy supplies:
Flavors, VG and PG
Note that nicotine is an addictive and dangerous chemical that should be handled with care, only
after educating yourself on the risks and procedures for handling it safely.
Reducing and ultimately eliminating nicotine from your mixes should be your goal.