These 3 vendors are the ones I use the most. They sell strictly for vape. Finding a few recipes is the way to go first. If you don't, as we have seen others just buy what they think looks good, then making a recipe often makes them have to order more for 1 or 2 ingredients they left off because they don't have what they need. It's best to know what you want to make (example Strawberry Milk, Blueberry Muffin, Strawberry Cheesecake, Unicorn Milk, etc. etc.) before you order.
http://www.bullcityvapor.com/ (Capella, Flavour Art, Flavor West, Hangsen, Inawera, Flavor Apprentice)
http://wizardlabs.us/ (Capella, LorAnn, Seedman, Flavor Apprentice, )
http://www.ecigexpress.com/ (Flavour Art, Flavor West, LorAnn, Flavors Express, Natures Flavors,
Signature (TFA), Jungle Flavors, Hangsen, Tasty Puff, Inawera, Capella)
When you ask about concentrated I don't know if we have the same understanding. Some brands are more concentrated than others, but to me they are all concentrated. I'm going to suggest you start with TFA and Capella, as they are the most similar in concentration. I can give you more info on TFA than I can the others. For example here is a tfa percentage chart :
http://www.v-ecigs.com/tfa-flavor-percentage-recommendations/
If you can say what juices you like or what flavors perhaps some of us can suggest recipes and look at how we can use the different brands successfully.