If you want more Warframe slots without spending Platinum, there are a few ways to go about it. First, you can buy Warframe packs from the in-game marketplace. These packs usually contain between one and three Warframe slots, and they can cost anywhere from 100 Platinum to 2,000 Platinum. Alternatively, you can spend real world money on Warframe boosters that will give you more slots. These boosters cost anywhere from 10 to 50 Platinum and they offer a variety of benefits, such as increased experience bonuses or faster progression through the game.
Finally, you can also purchase Warframe keys from the Steam marketplace. These keys allow you to unlock additional Warframe slots for use on your account, and they usually cost between $1 and $5. Overall, there are a number of ways to get more Warframe slots without spending Platinum. Just be sure to research each option carefully before making a purchase so that you know what benefits it offers and whether it is something that is worth your time and money.