Here are some of the special vehicles that i found on the PC that can be obtain without cheats or trainer on PS3/XBOX 360:
BP Trashmaster and Towtruck and UC2 Red Bison: http://gtaforums.com...s-quest/page-37
BP Barracks and Flatbed,UC Black and White Maverick(can be bought online),UC2 Olive Green Bison: http://gtaforums.com...s-quest/page-38
Small list of BP vehicles before unlocking Trevor and the colors from Al Di Napoli Vinewood Souvenirs: http://gtaforums.com...s-quest/page-39
UC2 Fusilade from Caida Libre is a Frost White Metallic with Sea Foam Blue Util Pearlscent http://gtaforums.com...s-quest/page-42
UC Packer,3 reused UP,2 PP Firetrucks and Ambulance: http://gtaforums.com...s-quest/page-43
PP tires Bison with the Winch from Marriage Counseling is obtainable with blue hell garage trick before entering the mission and doing the mission as normal,after the mission is passed the BP Bison is at the impound lot.