Gremlin DIY is always my first stop. They've usually got what I'm looking for and they're on the same coast as I am, so shipping is quick and I've yet to wait for more than 24 business day hours for my order to ship out from their shop.
If Gremlin doesn't have what I'm looking for, then my next stop is Bull City. They've got a HUGE selection, and carry some of the lesser used(? Still getting my mixing feet) flavor companies. They're located on the East Coast, so shipping takes an extra day or so, but they are also really fast about their order turn-around time.
I've just received my first orders from Nicotine River and Real Flavors. I'm not ready to offer a reasonable review on either vendor, save this: Both packages were well packed, with plenty of padding to ensure that items were protected. I made my orders from both companies during the Memorial Day sales, so there was an extra wait time for the orders to process. Shipping took a while because: A) In Nic River's case, the package had to ship via ground thanks to my ordering 100 mg/ml nicotine so FedEx took a while, and; B) In the case of Real Flavor, I selected the cheapest shipping option UPS Select. My package took three days to leave Ohio and a couple days to leave the sorting center in San Leandro, CA and then got hung up in Sacramento. To add insult to injury, the RF order was shoved into my mailbox and damn near required the Jaws of Life to remove. My Postal Carrier is a lazy jerk and is not getting any more XMas cookies, and is in no way, shape or form any fault of Real Flavors.