Need replacement earcuo for I Sony MDR-RF6000 but don't know how and where to buy extra earcup ?

You can get it from Microsoft

You could try Radio Shack.

Sony eSupport Electronics Purchase Replacement Parts Accessories You can find what you need here.

Try eBay and look for "Sony MDR-SA5000", that is SA and will work great as a replacement. You will find many vendors selling replacements for around 8 bucks. The vendors who are selling the exact replacements for the MDR-IF5000 are inferior to the original and cost more ($19.99). The SA replacements look exactly like the original and priced cheaper too, by about 1/3 the cost. They will be shipped from China, so if they offer free shipping could take up to 30 days to receive goods, just a heads up!

Reduce the volume level.

Check out My tip on headphones, at the bottom of it you will find a link to a place that should have them. Just click my user name to find the tip.

You could disassemble them and try to figure out whats wrong, and you may even be able to repair them, but headphones are fiddly and you'll need a multimeter and soldering iron at least.
A quick look on eBay shows a number of respectable looking sellers for around $15.
What is your time worth to you?

Yes, the replacement pad for mdr 7506 fits in for the v600. They can be replaced for one another. There are different types of v600 and 7506, You just have to look for the 7506 that looks more like the v600. That should do the job for you. Hope this information helped You.

https://servicesales.sel.sony.com/ecom/accessories/web/productSearch.do?searchOperation=modelSearch&searchText=MDRRF970RK&sessionId=FLk3TTjKj9RC2AYaSnUQb_k

This is the link from sony style for accessories and online ordering for your headset. Hope this helps.

You don't need to look inside the plug... just open up the earcups and test the jack plug with a continuity tester.

But FYI, Red = ring, Green = tip.