McMaster
The site isn't very user-friendly but they'll have what you need. To get to Neoprene tubing look for the "Tubing" link under "Fittings, Pipe, Tubing and Hose". Then under "Rubber", Neoprene will be listed. You can get it based on the Inner Diameter (ID) and outer Diameter (OD). Make sure you get the right ID based on the parts you're getting.
Hope that helps,
Indybird