go to any competent stereo shop and they can sell you speakers that fit. some can be cheap as hell, others really expensive.
Its up to you to decide on what you want and go listen to all the speakers that fit your car before you buy.
'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.