Here's the entire map of all collectibles:

And HERE is the same map in 4k resolution[i.imgur.com], patched together from screenshots.

Save this image on your computer and cross out the ones you have visited to keep track.

The ability to acquire barn finds is unlocked after you help set up any Horizon location.
The Jaguar Sport XJR-15 barn is only unlocked when setting up the Dirt Race HQ. So I'd recommend doing that one first.

The image above has numbers 1-14 written on it.
These cars are found at their corresponding number:

1. Renault 4L Export
   
2. Toyota #1 T100 Baja Truck
   
3. Ford Escort Racing MK1
   
4. Porsche Carrera 911 RS
   
5. BMW 2002 Turbo
   
6. Ford F-100
   
7. Ford Mustang GT 2+2 Fastback
   
8. Dodge Dart Hemi Superstock
   
9. GMC Jimmy
   
10. 95' Chevrolet Corvette
    
11. Dodge Viper GTS ACR
    
12. Ferrari 250 GTO
    
13. Ferrari F40 Competizione
    
14. Jaguar Sport XJR-15
    (Unlocked only after setting up Horizon Dirt)