Wizard labs for nic and VG bases...best prices usually etc. (and myfreedomsmokes when on sale) for nicotine bases. RTS vapes often has sales on VG or PG etc.
For DIY flavor extracts, if you're into tobaccos, would be from naturally-extracted-tobacco.com
Vapingzones super concentrated alcohol based flavors, Nature'sFlavoursOrganics (from highdessertvapes and ecigexpress) and some FlavourArt flavors from them as well. Those because there's no PG in them and therefore my favorites. I only need from 2 to 5% in my DIY e-liquids, but that's just me as I've heard many people like 20%!
There are some nice flavors that are PG based from TPA/TFA but I can only use them at less than or up to 1% (which is actually a good thing as far as not having to re-buy very often since they'll last me a few years lol.) Lot's of vendors carry those, just watch for sales.
If you're into WTA for DIY, it would have to be wholecig.com for pricing and best "taste" of the "unflavored 100%VG" (without PG added).
Good luck