had the same problem, if you don't want a full system built in to the car (i like my cars original look)
rear speakers:
http://www.blaupunktdirect.co.uk/speakers/incar-series-speakers/ic-109.htmlthey fit right in,
front speakers (87mm), i bought the smallest from blaupunkt, (
http://www.blaupunktdirect.co.uk/speakers/gt-silver-cone-speakers/gtx-352-sc.html)
but they are to "high" and the caps did not close anymore, fixed it by filing of the plastic untill it fitted
(a 5 minute job /speaker)
take note, if you want BASS, you won't be happy by the results of this setup,
radio etc it goes fine: even when played rather loud it stays clear, however: lower freq music (expl techno, house ...) is not optimal ... not so great, but more then acceptable for me.