Download the MP3 from where? If you want a copy of a well known song, you might find one, but for something else you might struggle to find one. Your best bet is to buy used CDs and rip them yourself.
My kid is only 2 years old, but when he can understand the concept I will be sure to teach him that he doesn't have an arbitrarily large choice, he can only choose from within the options we have available.