They could be the Sims 3 Store
Yoga pants. I had a look in-game and those look very similar when the sim is wearing them. You can't see it in the preview image in the Store but they do have that little split on the side, too. Admittedly the pants in the Soslilliom preview image look like they are slightly narrower in the leg, but if you can't find those exact ones the yoga pants would make a fairly good substitute.