CS:GO case keys?

Not likely. It's being used as a currency for trading, so it's usually always the same price.
You can try in the trades section of Steamgifts.

Or try the trade section from SG or I would recommend OPSkins, it's a trustworthy store, price there is just a bit lower than Market, but no week cooldown from trading... If you're going to buy from some other trader, be careful, check reps, SG, and or any other trade community profile and such, mostly the 'trading 101'.

1. buy them on the market
2. buy them ingam

Try CsgoOP.