It's part of the Plymouth DLC so you will need it in order to buy the part from the parts-store. Or you could find another S15 at the scrap jard and hope you can repair the part to 100%.

I have all the DLC's and still can't find it. Help me please.

This part is supposed to be an "AC Condenser". Although it's possible to add this in the car editor, it doesn't actually exist. It's not part of any dlc! That's why you can't buy it.
All you can do is remove it from the mod, which is actually not that hard:

• go to the folder
  
  ... \SteamLibrary\steamapps\workshop\content\645630\1309388129
• open the file config.txt with a text-editor
  
• look for the paragraph
  
  [parts5] name=#ACCondenser position=-0.0122,0.3067,1.5033 reflection=False rotation=1,180,0 scale=1.1
  and remove it.
  
• save the file and repeat this with the other config-files (config1 to config4).

Very Cool , I will try that , Thank A million, :)

I forgot to mention something, that may or may not be important:
after removing
from the file, make sure, that the remaining [parts] are numbered consecutively!
If there's a [parts6] or [parts7], rename them 5 and 6 respectively.

I don't know, if this is crucial, but better safe than sorry.

If you use the 'Car Editor' from the 'Play' button in steam to remove the offending 'part' it will renumber the parts properly for you. Otherwise, doing it manually, yes you will have to re-number them appropriately. However, I've not experimented with that to see if it is crucial or not.