I just bought mine from http://www.aliexpress.com
. Don't buy from any other vendor/seller/dealer on that website except for: UNIT PACK POWER (UPP). Their product is genuine, they answered all my questions, and customized my battery - I asked for them to install a BMS with a higher amp rating.
It took 11 days from China from the shipping date to receive it here in California. I opened up the pack at once to inspect and made sure the batteries where genuine Sanyo 18650GA's as was advertised in the item listing.
I didn't want to buy from China or overseas, but after reading the feedback left by other people who have bought their (UPP) batteries, and reading the product descriptions and comparing them to the other sellers on aliexpress, I went with that company. The other companies seem flaky.
I made sure I wasn't going to through away my money by having a battery pack made of generic/fake batteries. Most of the dealers/companies on there don't state what batteries they are using.
If you go through aliexpress.com and UPP, be very patient...it took about 3-5 days for my credit card to be verified (a good security feature of aliexpress) on top of the 11 days waiting for the battery.