It looks like McMaster-Carr is selling the appropriate type of flexible silicone tubing.
Unfortunately it is difficult to link directly to any product, so you'll have to look around for yourself. There are a lot of choices!
This is the item that looked approptiate to me:
Silicone Rubber Tubing Soft, 1/8" Inner diameter, 1/4" Outer diameter, 1/16" Wall, White
1-99 Ft.: $0.56 per Ft.
Hope that helps.