5x7 and 6x8 are usually the same speaker...just different mounting points. Most 5x7 speakers also have mounts for it to be 6x8's, as far as the grill...you can search around the net or go to a local audio shop to see what they have. Sometimes you can get lucky and find somewhere that has a few laying around they'll give for cheap. I got 6x9 grills for my old car for 5 bucks a pair. They weren't anything special, but covered the hole in my dash.