I honestly never though it would come to this. Dressing up in a duck mask and a blue robe for Buddha's Birthday was one thing, but by actually purchasing yoga pants, i've clearly crossed a line.
While I'm a little surprised at my purchase, for sitting zazen you need something that's not flashy, covers the legs, doesn't constrict and will be comfortable for the entire day. Yoga pants with a pair of compression shorts fit the bill and are easily obtained at the local sporting goods shoppe.
Traditional monks robes, or koromo, are rather expensive, take time to acquire and somewhat difficult to wear, though they are very well suited to sitting. Doing ōryōki for the first time should be sufficiently challenging, no need to further complicate things.