I LOVE athleisure! Yoga Outlet is great for discount leggings, tops, and other yoga gear (Alo Yoga, Onzie, Vimmia, Korel); Athleta has good quality and cute athleisure wear as well. Jcrew has started selling Outdoor Voices (which is supposed to be the new “Lululemon” minus the insulting CEO).
If you want to try a service, Ellie and Fabletics are the known ones to help stock up (monthly box for $50), but it is near impossible to quit Fabletics, so unless you want membership for life, I suggest avoiding that one.
My favorite sites are Carbon38 and Spiritual Gangster; the pricing can be steep, but they both have good sales.