In the online lessons you can get a complete list of recommendations for materials with the online links to buy. But here is my recommendation if you are running short of watercolour paper. The watercolour pads are good value and at Jacksons I have picked out this one here
Pads are a great idea, but I always recommend to take out the sheets and place on a board and then tape (free setting up online video shows you how to do this)
Jacksons Art Supplies are one of the online art supplies that are managing to keep going, albeit with slower delivery times but nevertheless!