Importing from China - If you buy on PayPal you have some safety. Lets say for example you buy and they dont declare and it gets caught, product does not come in a timely manner and you get a refund. Flip-side is they dont declore and it slides on by, you are fine.
PayPal + eBay = even more safety in your purchase. I cant think of any online stores without going to my MS Word document, but places like Fast Tech, DX, Battery Supports, AllBattery, you could also try electricmotorsport, or Luna, or Kinaye, or if you want something RIGHT NOW, buy a Home Depot battery, ~50V 7.5Ah good to go.
Since the other guy is in Canada, why don't you support Justin at www.ebikes.ca he sells top notch quality products and is backed with superb service, warranties and guarantee's. He will not ship undeclared batteries so you will pay a little more but its all worth it. I forget how much extra it would be, I would expect $50 Hasmat Fee, plus S/H.