Very cheap and trusted is a bit of a contradiction in terms. But I'd say Amazon.com is one of the best places to shop for electronics that you can't find at your local store, so long as you use some common sense and buy from highly rated sources. I've purchased many items from them with very few problems. For example, I never buy from sources with less than a 95% feedback score. Also, look at that particular product's star rating as this will give you some clue as to other buyers' experiences. Finally, if you buy directly from Amazon (not an affiliated store) they give you a great return policy. Otherwise, you must deal with that particular store directly, which can be more difficult if you have a problem.
Hope that helps.