You used to be able to buy pet slots with sp, is that still allowed? I didn't see it as an option.

It's available in the Millionaire Center in Centropolis. The first slot costs 20m sP and they increase by 20m per slot up until the 10th (and currently final) slot at 200m sP.

You can also buy CSC cards with sP from user shops or through trades from other users and use those to buy your CSC pet slots: