nos parts

napa,az,oboys,aap or wherever.

when buying replacement parts, when buying ahead, as in I don't need the part tomorrow, I tend to price shop between AZ, RA and NAPA, I include the shipping price when considering RA,, It is surprising how the price can range between the three. I usually go with the cheapest, unless the AZ part sports their Duralast Lift Time warranty. Then I go there. I put a Duralast power steering pump on my 83 D150 in 2010, it started dripping oil this past spring ( nine years later) I went back at AZ,
I did not have the receipt and they looked the purchase up in their system and honored the warranty.

If the part is something that is not available, like the inside the dash air conditioning ducts for the same D150, it is prowling around the local pull a parts.
Forums like FABO are also a great source for hard to find parts, then there are swap meets local and the big ones like Carlisle.
Another source is large internet connections like E bay, craigs list and www.car-part.com