Chocolate frogs can be found in Honeydukes, which is accessed through Hogsmeade. To gain access to Hogsmeade, you need a Maurader's Map which costs 10 galleons. Other ways you can get chocolate frogs is by trading them with other people; I know people will trade chocolate frogs for other things (including money) as well as giving them away for free if they have duplicates! There is a chocolate frog trading thread as well.

@Donnovan Perrywinkle

Actually, you need the fake permission form to gain access to Hogsmeade. It used to be the Marauder's map but that has changed recently - now the Marauder's map shows online users. The fake permission form can be bought for 5G in Weasley's Wizarding Wheezes. Joining our discord is a quick way of finding others to trade cards with. Have fun!