Sierra Trading Post was mentioned. Also January sales are going on right now at most places like Ex Officio, Columbia, Eddie Bauer, North Face, Marmot, etc. You will need to go to each individual web site. Go to REI and Look at the brands for an idea of what is available. Then head over to the brand website.
Sometimes I find good stuff at much reduced prices on eBay. This is especially true for some of my now discontinued favorite items.
REI garage is a good place on occasion. People buy stuff, use it once, and it goes into the garage.
Also look at REI Outlet.
Stanford may have a resale exchange somewhere.
Target golf pants and golf shirts aren’t bad, as well as Champion undies.
Uniqlo has some great stuff. Heattech and Airism underlayers. Cheap down jackets and raincoats.
Sometimes I find light shirts at H&M.
Decathlon has a store in San Francisco.
Here is a good article on budget sources.
Your goal is light weight and quick drying. You can layer for warmth.
Invest in good shoes and good rainwear. Most other things can be of lesser quality.
In general I go for one lightweight pant minimum and one lightweight shirt minimum. Then I can add in from my regular clothes. As I’ve aged. I’ve made sure that most of my clothing are also travel worthy.