Some of them may be available for listening at big chain stores like circuit city, etc. Otherwise, most of the manufacturers have web pages listed at http://www.hifiheaven.com/car-speakers.htm.
These sites usually have a dealer location feature based on your zip code. Diamond audio isn't listed on this page, they are at http://www.diamondaudio.com/ and you have to email them to get an authorized dealer list - their warranty is doubled if a dealer installs them.

__________________
John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E