Anyone who is in the hobby long enough eventually ends up ordering something from HobbyKing, an online hobby store based in Hong Kong. Assuming you are willing to wait a few weeks for a shipment (they are in Asia), HobbyKing is by far the cheapest source for batteries, chargers, connectors, and even feature-laden radios and other products. Hobby King has standardized around the XT60 2-pin, Nylon battery connector (top photo). The connector was designed by hexTronic, who have deliberately not filed a patent on its design in order to encourage proliferation.
I love the XT60 connector. It is inexpensive (5 pairs for $3.19 + shipping), connects securely, is easy to solder, and is not easy to short out because all exposed metal is recessed and/or designed to be covered by heat shrink tubing. HobbyKing claims that the XT60 connector can handle 60A for extended periods of time without exceeding 80ºC, and the nylon XT60 connectors I have purchased directly from them have all been high in quality—I haven’t had to replace a single connector due to poor manufacturing. Highly recommended.