Buying online can be done from shops or directly from other collectors. Shops (Like Ours) are independent and profit margins are often much lower than a mass retailer. Independent shops often come recommended by other collectors but can often be found by a google search. Mass retailers such as Game or HMV will often charge more per pack (unless on promotion). Online trade sites such as EBay and Vinted can be used, with caution.
You can find trading, buying and selling groups online. People often show and sell their personal collections, transferring money via PayPal or eBay lot set up. When buying via PayPal, you are only covered if you pay via goods and services payment.