If you are looking to find authentic car parts to restore a vehicle, the biggest challenge in finding something authentic is in making sure it is not just a reproduction. There are some clues that are dead giveaways to finding car parts true to the period of the vehicle that you are attempting to restore. For the sake of argument here, let's say you are looking for a 1970-74 E body dash pad for the dash of your classic vehicle. The following telltale signs are as follows.
In this narrow window of time, an E body dash pad only came in six specific colors. Two shades of blue, dark red, black, brown, and dark green should be your only choices. If the dash pad someone is trying to sell you is any other color but, it is a reproduction and you should refuse it. The dash pads of this era were often designed to match both the interior and exterior of the vehicles, so alternative colors are not authentic vintage parts.
If you are old enough to remember sitting in the front seat of a vehicle in the early seventies, then you are probably old enough to remember what the dashes felt like. Most of them had tiny raised bumps all over really hard plastic. It was akin to touching the warty skin of a young toad but very hard and not at all soft. Although some reproductions have managed to capture this texture in a similar way, you should be looking for small grain bumps on very thick, very hard plastic, not thin plastic.
The Number of Pieces
Dashes in the seventies went for the singular piece; no breaks, no extra pieces or seams besides those around the radio and steering column. The authentic ones will be one piece, very heavy, and easy to scratch with a pin. The reproductions cannot be scratched at all because they are made from a very different sort of plastic that has been modernized and made to be lighter in weight. If someone tries to sell you an E dash pad that is more than one piece, has multiple seams, and/or can be carried by a ten-year-old with little effort, that is not an authentic vintage part.
Your Best Bet
Visit classic and vintage car shows. You would be surprised to see all of the car parts for sale. Sellers are often car restoring hobbyists like yourself, and they can give you more pointers on differences between an authentic and reproduction 1970-74 E Body Dash Pad.