I usually either stalk really high end yoga pants brands on poshmark and ebay (Alo Yoga, for example), or I get Popflex brand stuff, which has lots of pockets and nice fabric but a much more reasonable price point than say, lululemon.
I try to buy nothing new, but I did get a LOT of stuff from ebay. I work out every day and worked in a high-end gym and hate doing laundry constantly, so I have almost a full week’s worth of leggings and sports bras, etc. I would also say that I’m extremely lucky as I live in Portland which has the headquarters for Nike and Adidas, so there’s an abundance of high-end athletic wear in the goodwill here.
If I want merino wool (for travel or bike touring) I stalk icebreaker on ebay or on backcountry.com