You can get underbody deadener from Autobarn - fairly confident that other similar places would have it too. Car sound places also sell stuff like Stinger Road Kill, Jaycar sell brown bread - there's al sorts of stuff - depends on what you want to use - ie bitumen type paint on or spray on stuff or sheets of sticky bitumen that come in a roll opposed to a tin or can. Go down to jaycar and see what's best suited to your application - it all varies on price too obviously.
I'm looking at doing my doors - I want to try and reduce the amount of road noise - it absoultely $hits me every freakin day LOL ! I'm thinking of using a paint on bitumen type, then using rolls of matting afterwards - the general consensus is the more layers the better...but therefore also more expensive, and it can add a lot of weight to your car. Just to give you an idea - a guy on another forum sound deadened his entire car (underneath floor carpet, under back seats, boot, doors, roof - the sound deadening weighed in at 80 KG's alone ! Ok, that's my 2c =]
Wagons are rarer ;-)