I needed an 82mm replacement lens cap for my Sigma 100-300 f4. Preferably, the center pinch type since the one provided by Sigma can’t be removed or attached with the lens hood on. 
I found a really inexpensive version on eBay and decided to give it a try. It works great!
Search this seller’s store for 82mm:
http://search.ebay.com/_W0QQsassZjiakgong
I paid $3.60 with free shipping. It took 7 days to reach me in the US, although I assume transit times can vary widely.
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Don 10.04.10 at 12:28 pm
I was in the same boat since the old style cap pretty much sucked. I contacted Sigma and they offered to send me a new cap free of charge! Excellent customer service! They even sent me a freebie lens cleaning cloth as well.