CSGO Lounge - Marketplace, Trades, Bets is the main site where people bet skins on matches. You can also trade there.
You choose a match and it'll list out your inventory. Select the items you want to bet and then one of their bots will send you a trade offer.
I usually buy keys from my friends or some trusted sellers from /r/globaloffensivetrade. It's a cheaper and the items are not trade locked but you do run the risk of getting scammed when you deal directly with the person.