You can't buy the lemonade concentrate anywhere since it's by Starbucks and they don't sell it.
The closest version is Minute Maid Lemonade or Simply Lemonade.
Lemonade is great in iced AND hot teas. Try steamed lemonade in hot teas like Mint Majesty, Peach Tranquility, or Jade Citrus Mint.
Or order their popular Honey Citrus Mint Tea which is made with steamed lemonade.
Frozen lemonade drinks that are frothy and slushy can be made at Starbucks by asking for the Lemonade to be double blended. Ice and lemonade are put into a blender until smooth.