You can often find physical, sealed "New Old Stock" DVDs on eBay or computer liquidation sites. This is the closest you can get to a "legitimate" retail experience. Look for a COA (Certificate of Authenticity) sticker to ensure the key hasn't been reused.
Sometimes it's easier to buy the license with a computer attached. Searching for "new old stock" business laptops (like old Dell Latitudes or Lenovo ThinkPads) can land you a machine that still has a valid Windows 7 Pro license built-in. Hunting for the Software (ISOs)
Because Microsoft and authorized retailers no longer stock Windows 7, you cannot buy it directly. Your options now depend on how much "adventure" you want: