Here:
Banshee - Find this on Galveston Ave right by Middle Park. Based on Dodge Viper.
Comet - Find this between the Burger Shot and the Swingers Golf Club
Coquette - Find this one outside the church
Infernus - Find this in Outlook Park by the Pizza Salad restaurant
Super GT - Find this on Calcium Street in front of the 69
Turismo - Find one parked outside a home in Owl Creek Avenue in Westdyke, or next to Faustin's house on Shinnecock Ave in Beachgate, Broker
Rebla - You can find this by Fanny Crab's near Grand Easton Station.
Sentinel - Find this car parked by the docks in Normandy
Sultan RS - Find this behind the garages of the deserted house in Westdyke. house in Berchem.
Admiral - Find this in the Meadows Park, Dukes district
Hakumai - Find in Steinway, Dukes
Dilettante - Find this by the Canyon Magaplex in Willis, Dukes.
Lokus - Find in East Island City neighborhood in Dukes
PMP 600 - Find this in the alley by the Exchange District Bank in Algonquin
Washington - Find this by the pool in Steinway, Dukes
DF8-90 - Find this by the Amusement Park parking lot.
Sultan - Find this in the driveway of a green house in Berchem.
Bikes
NRG 900 - Find this on the corner of Bismark Ave & Topaz in Lancaster
Helis
Helitours Maverick - Found at the Helitours company in Fishmarket South.
Police Maverick - Found on top of the police station in Algonquin
Boats
Dinghy - Large dinghy with four seats and two big outboard motors. Several can be found around the coast of Acter Industrial Park.
Reefer - Motor cruiser with cabin. Several can be found around the coast of Acter Industrial Park.
Special or Rare vehicles
Mr. Tasty - Ice cream truck. Found around Broker and Dukes.
Sultan RS - Find this street-racer car behind the garages of the deserted house at the end of the wavy track in north-east Northwood.
Heavy Trucks, ect
Fork Lift - Can be found in south-east Happiness Island or around the docks in Port Tudor. Occassionally on the street to the left of the bridge under construction in Bohan.
Rancher - Find them down by the docks in Port Tudor.
Air Tug - Find it at the airport.
Phantom - Cab of an 18 wheeler. Find outside of the 1st floor of lawers office GL&S
Lowriders
Manana - Find this by the Burger Shot on Huntington Street.
Voodoo - Find this in East Island City.
It's all what I have.
Source: http://grandtheftauto.ign.com/wiki/GTA_IV_Vehicles