Just trade for 12 plat and you got yourself 2 weapon slots.
Also, there is no pyramid, unless if you're actually THAT impatient (In that case, Warframe probably isn't the kind of game you should be playing). Friends can gift you weapon slots and weapons via the Markets gifting featur, which will grant you two weapon slots or a single weapon slot with a weapon already occupying it respectively. As of now, you can get a Dex Sybaris, Dex Akfuris, and Dex Dakra (Not recommended to sell unless if you already have duplicates of them).
You can also get free weapon slots from Dex Weapons when Warframe's Anniversary comes around.
The only way to get free weapon slots is by getting the Azima or Zenistar from logging in (Not recommended to sell because they're obtainable once, and only once) and getting Broken War from The Second Dream and Broken Scepter from The War Within (Also not recommended to sell for the same reasons).
