I've bought and used 100mg nic base from the following suppliers: Nude Nicotine, Wizard Labs, Heartland Vapes, RTS Vapes (now Nicotine Giant), Liquid Nicotine Wholesalers, and My Freedom Smokes.
The nic base from the first 5 of those was very good, IMO, though the cost of it from the first 2 was quite a bit higher than from the rest. So, the liters of 100mg nic in "my freezer stash" for the longhaul are from Heartland Vapes, Nicotine Giant, and Liquid Nicotine Wholesalers.
The only 1 of those 6 suppliers that I will never
buy nic base from again is My Freedom Smokes. It was very "peppery" and harsh
to me, and they shorted me 180mL of the Liter. I gave some of it away and sold the rest to a local DIYer who liked its peppery characteristic, since he prefers tobacco DIY eliquids.
ETA: As far as speed of shipping it once I'd ordered it...all shipped within 1-1.5 business days, except for Nude Nicotine. They were very slow compared to the others.