They're on eBay of course, but after spending a lot of time massaging one to make it fit my 520 chain, it ended up not working. They sell them as a "520 half link" on eBay, but they're labeled as being for #50 chain on their packaging. After spending way too much time on one the other day trying to make it fit, I went to the hardware store looking for something else and I happened to see that they sold the very same links at the hardware store that I had gotten from eBay. The hardware store sells them as #50, not 520, which are two different chain sizes. The eBay sellers selling these links as 520 are WRONG because the #50 links WILL NOT WORK on a 520 chain.
I ended up having to buy a true 520 half link from Europe, and it's the only true 520 half link I've ever seen, assuming it is correct once it arrives. It looks good in the listing though.