http://www.ebairsoft.com/jing-gong-body-p-1258.html
Here is the endcap replacement only. I don't know if it comes with with the metal loops though!

If you want one that defintely has the metal loops, I have a spare MP5-J retractable stock set with the stock and endcap. I can sell you mine for $35 + shipping but unfortunately, I want to keep it together and can't sell you only the the loops.