As Helinut says, your school will give you a list. As for where, there are a number of websites, does the local flying club sell what you need? Often with small orders, when you add postage etc., the local club is as close as makes no different in the aviation world. Support them buy getting the stuff from them.
Some schools also have a discount code you can pop into a suppliers website, which works for some routine items, if they don't sell what you need.