from what I recall, these end caps were in a kit made by Strut...and other than that...the cheapest ones would be ebay.
Personally, I wouldn't even spend $300 on ebay for them...I know for a fact...it probably will cost you even more to find a set that is made by Strut....have to go thru a vendor...then you have to get it painted...etc.
Good luck in your search.