Vapable is OK but expensive. I use Darkstar. They sell 2L of nicotine for about £70.
Most people get a bunch of nic and stock, but keep in mind that you won't be able to keep it forever. Nicotine gets bad easily.
If you store it, preferably freeze it and top it up with something like argon gas so the nicotine won't oxidize with oxygen.
After that, you should still be able to buy 20mg strength nicotine. You'll still be able to do DIY, but obviously you'll need to use more liquid of your nicotine base and you'll never get more than 20mg strength (which should be more than enough for any mod).