I traced my crank no start issue to the ECM. Is there a vendor where can buy a cap kit to replace failing capacitors?

 

Digi-key or Mouser.

Youll have to get the part numbers off of the old caps.

 

Depending on what year and model car you have, there are some vendors on ebay that gathered capacitor kits for sale.

 

As mentioned above. Mouser, Digikey or even Jameco. Having done this before, I would get the numbers off the individual caps themselves and then goto a slightly higher voltage rating on the cap, like if its 25v, 35v or 50v will work fine as long as the capacitances are the same. By doing it this way you have some extra just in case.