With my cards not currently in a deck I put them all in money bags you can get from a bank, like, small plastic bags. Shops also use them when they put change in the tills. They are a pretty good fit. The worst thing that could happen would be the front of a card getting scratched from the back of another, which I've never experienced or heard of. It keeps the cards rigid and together nicely. I also bought storage boxes from amazon. Just small ones. I think it was a pack of 5 which hold "1000" cards cost £12 ($20, €17 roughly). They actually hold closer to 1200.
The bags idea is also good for sorting cards. I have my white creatures in one package, other whites in another, etc. It's easy to find the card I want