i highly recommend http://www.talkaudio.co.uk to do some serious research on this... there is a wealth of information/tips on there.
if it is road noise you are trying to eliminate then sound deadening wont make a huge amount of difference, if any... sound deadening stops the vibrations in the panels caused by the vibes from speakers etc to keep the sound in the car. there are products available to filter the noise coming from the road/exhaust etc that is placed on top of the sound deadening under the carpet... in fact there are so many different products available that, by the time you have finished, your carpet will be up to your steering column, your car will weigh the same as the ss enterprise and you will have remortgaged your house!
for buying spares i will highly recommend Car Audio Direct ... after days and hours of research the second skin damplifier is what i invested in for sound deadening as it is alot cheaper than dynamat and comes out far superior in performance. i personally avoided any cheap alternatives after reading many nightmare stories of aftermath results! as for noise barriers noise barriers/filters, there are several options available for different jobs but are shockingly expensive and they are essentially just sheets of closed cell foam... very much the same as the roll up mats used for camping! i contacted cad to discuss this and was told that it is essentially the same material but their products have been 'tuned' to the specific frequencies of road noise. i bought some rolls of camping mat from ebay for less than a tenner to see for myself but have yet to fully blitz the interior.
