The typical costume during San Fermin is the (almost) obligatory white and red.  Everyone, from babies to grandparents, is dressed in all white with a red pañuelo (handkerchief) around the neck and a red faja (sash) around the waist. 
This costume is easy enough to improvise, however almost every street has several shops selling the traditional white pants/skirt and white t-shirts.  The white cotton pants cost less than 10 euros and a t-shirt often costs less than five euros. The pañuelo and faja can be purchased for a few euros each.  Complete packages can also be purchased for about 20 euros.