Uniqlo and Muji are probably regarded as the gold standard around here for inexpensive decent quality basics. Belle + Canvas has some great t-shirts, too. Check out r/frugalmalefasion for some deals, too. Keep in mind FMF is somewhat more of a male fashion sales/deals sub, but it has some frugal stuff too. More on reddit.com

Costco is better than the alternatives that you listed. Kohls is also ok when there are big sales and bonuses. In general, the goal of being frugal is to get the best value out of your money. This means the cheapest option is often not the best. Despite price increases, Goodwill may still be more frugal than poor quality new clothing. Some name brand clothing is actually higher quality and may be the frugal choice in the long run. More on reddit.com