I am looking for vehicles from the 1970's era. What is a good source?
You might try Truckstop Models
http://truckstopmodels.com/
and Model Train Stuff (MB Klein Hobbies)
http://www.modeltrainstuff.com/
and Buy Hobbies (distributor for Classic Metalworks)
http://store.buy-hobbies.com/automobiles.aspx?page=4
and Burbank House of Hobbies
http://www.houseofhobbies.com/hovehicles.html
Between them, you should find what you're looking for in 1/87 scale. A few you might look for include the following:
Classic Metalworks: 78 Chevy Impala sedans and police cars (if you're lucky you might run across their discontinued 1970 Chevelle SS coupe as well).
Roco: 75 Dodge pickup & tow truck
Ricko: 71 Plymouth Cuda convertible
Busch: 78 Dodge Monaco and Plymouth Fury 4 door sedans and police cars & 74 Pontiac Trans Am
Fresh Cherries (Motor Max): 71 Mustang, Maverick coupe, Pinto wagon, Mercury Bobcat, 75 Ford Granada, Ford Tempo and Escort, 74 AMC Hornet hatchback, Gremlin, Pacer, late 70's Honda Accord and Civic
Masterpiece: 1970 Ford Galaxie 4 door sedan and police car, 60's-80's style step van
Herpa: 79 Mustang, late 70's-early 80's Buick Grand National, 69-70 Mustang (has 69 grille and 70 tail lights)
Trident: Chevy vans and pickup from the late 70's-early 80's
Brekina: 71 Chevy Camaro, 64-70 style Dodge A-100 vans, 68-72 style Chevy Corvettes
Model Power: 70 Plymouth Cuda coupe
Reel Rides: 72 Chevy pickup, 70 Pontiac GTO Judge, 79 El Dorado
Atlas: 78 Ford Fairmont 4 door sedans and station wagons
Williams Brothers: late 70's Chevy El Camino (clear plastic kit)
and if you want European cars from the 70's, try Wiking, Herpa, Ricko, Busch, Brekina and probably a few others.
60's American cars are a more neglected era for 1/87 scale, but there are a few out there and they wouldn't look out of place in the 70's. Herpa has a 63 Corvette Stingray, Busch, a nice 65 Mustang convertible and hardtop and 66 Cadillac limos, ambulances and hearses, Ricko has a nice 63 Lincoln Continental convertible (the ones with the suicide doors), Model Power has a 68 Chevy El Camino, 63 Thunderbird, 69 Camaro convertible, 69 Olds 442, Masterpiece has a 68 Plymouth Fury sedan and police car (discontinued), Classic Metalworks a 66-67 Dodge Charger, 67 Ford Galaxie 4 door sedans, police cars and taxis, and 61 Chevy Impala coupe, Reel Rides a 67 Plymouth GTX convertible, and Williams Brothers a 64 Pontiac GTO coupe (clear plastic kit). More difficult to find but worth seaching for is the discontinued Walthers 66 Ford pickup (Walthers also made a 67 Mustang fastback), and Road Ragers for a 60 Ford Falcon 4 door sedan, 61 Falcon wagon, 67 Falcon 4 door sedan.
For 70's era trucks & buses, there are several that come to mind, including Athearn, Trucks N Stuff, Herpa, Con-Cor, Busch (for a transit bus), to start with.