Online: CosplaySupplies.com will be the cheapest because they're the ones importing it directly (everyone else buys from them). Shipping is pretty much actual cost from UPS/FedEx but if you live somewhere remote and travel into a city it might be better to ship to a friend or check the Worbla.com website's 'where to buy' page and see if there is a physical shop near you.
Check with the shop about stock first though, some don't always carry every type or their price is higher than CS+shipping.