Depending on the brand of your equipment, you can find air compressor parts at the store that has provided you the air compressor. If you are fine with parts from previously used equipment, I suggest you contact a used parts provider in your local area.
I wouldn’t go as far as trying to replace air conditioning parts for my car by myself simply because the air conditioning system is quite complex. I recommend you go to a car specialist and have the replacements done because you also have the guarantee that it would work.
If you make sure the used air compressor parts fit your air compressor fine and they are not damaged in any way, yes, I recommend you buy them. It often happens that people cannot find the proper parts because they do not manufacture them anymore so used parts are a good alternative.
Well, if you don’t have the original instructions and you have no manuals on air compressors I would recommend going online to a dealer or manufacturer of air compressor parts and obtaining a parts list form them.
I would recommend you go straight to the assembly instructions and lay out the air compressor parts in the order you will need to assemble them so you can become familiar with them. Study the instructions to see which sections of the assembly you are confident you can perform. Maybe you can call a buddy of yours and have him come over and help you put it together. There ought to be a help line listed in the instructions, if not check out their website just in case you need help.
You shouldn’t have a problem as long as the manufacturer of the air compressor used standard parts and design. Most of the parts you may need can be purchase through alternative manufacturers. I would recommend you have the model number and specifications available when you contact either a dealer or manufacturer. Check out their websites and you can save a little of effort.