Here you can find Pouches for Russian and NATO flasks. They were created to provide ability to put flask into pouch and place it on vest or belt.
Here you can find pouches for hydration systems, like Source one and similar one. It's very useful, because you can drink water during combat.
Here you can find special pouches for bottles or thermos. Prefer to drink from standard 0.6 liters bottles your favorite Coca-Cola or Pepsi? This is the best decision!