What kind of convex washer goes on the brake pads? I forgot to tighten up my brake pads before I went for a ride today, and I lost the convex washers when the nut came loose? Could anyone tell me where I could find replacement washers for the brake pad? I checked at lowes and attempted to google them, but I have no idea how these washers are sized.
Buy yourself a set of new brake pads.  They come with all the washers and nuts.  When the current pads wear out, move all the washers and bolts to the newer pads, which you keep safe till you need them. 

Or just fit new pads now.