I get about 80% of mine from Wizard Labs, and 20% from OSDIY. I've gotten some in the past from 2 other vendors as well, and I wasn't impressed at all with them.
I like WL because they carry so many different manufacturers' flavorings, their price for most 8mL sample sizes is $1.49, and their Customer Service and packing for shipping are second to none! Also, for TFA flavorings...their prices beat even ordering directly from TFA for up to and including the 4oz. bottle size!
I love OSDIY Shop because of their One Stop Flavors and their One Stop Blends mainly, but while I'm ordering them I usually toss in a few TFA and others I might be running low on, too. They're a wonderful, family owned/run company, have great customer service, and always have an ongoing coupon code for anywhere from 10-15% off usually, depending on what's already on sale. The codes are ECF or JFK for the discount.