Where do I buy Warframe slots?

I'm curious, could you please explain to me where exactly can I purchase Warframe slots? I've been hearing a lot about the game and its expanding inventory system, but I'm not sure where to find these slots to enhance my gameplay experience. Is there a specific in-game store or marketplace where I can make the purchase? Additionally, are there any tips or strategies you could recommend to optimize my spending on these slots? I'd greatly appreciate any guidance you can provide on this matter.