It depends a lot on your budget. Thrift stores are great places to find hidden gems for cheap, especially if you have some good stores in your area. If you have more to spend, like modern styles and you aren’t up for a custom suit, then I would recommend places like Express where you can find a three-piece suit for a decent price (especially with their regular specials/discounts/sales). Indochino is a good place to start for a custom three-piece, and while I haven’t shopped there myself, I’ve heard good things. Hope that helps some!
At the moment I own 7 three-piece suits, 6 two piece suits, 3 blazers, and 3 pant/vest combos. And I suppose my closet is pretty big.