I usually buy nic base from Heartland Vapes and Liquid Nicotine Wholesalers. Both have very good nic base, IMO, and they carry a wide range of mg, too. Heartland is less expensive, but with the coupon code ELRECIPES (which is an ongoing 15% off coupon to use for anything sitewide), Liquid Nicotine Wholesalers comes within about $3-4 of Heartland's $44.99 per Liter price.
For flavorings, my 2 favorite places are One Stop DIY Shop and Bull City Flavors. OSDIY has an ongoing 12% off coupon code for ECF members, which is simply ECF. I really like many of their own One Stop Flavors and One Stop Blends, as well as the other major brand flavorings they carry. They frequently run sales with higher % off at major holiday times. Bull City carries some flavoring brands that OSDIY doesn't, and they have an ongoing 6% off code for ECF members. It's ECF6. Both have excellent customer service, fast shipping times, and fair prices, to me.