i doubt an automotive store will have them. head over to lowe's or home depot, pick up some 3/4" birch plywood (the small 2x2 sheet will be fine) and make some yourself. it'll only take 10 minutes to make them and it's cheaper than buying them premade.
what speakers did you put in? some speakers will need to have 2 rings, for a 1.5" total spacer, made for extra clearance.