Saddle bags for a 2003 Ducati Monster 620

Anyone know of where I can buy saddle bags or any type of bag for my 2003 ducati monster 620? I would like it to be a decent size to at least hold maybe food, a beach towel, some travel items. Im not looking for anything too pricy. Also, would it be ok to maybe use a regular backpack? If so, what kind do you recommend?

Site: her-motorcycle.com