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:
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.