There's a lot of manufacturers that make them. If you google "Solid rings for fishing" they will all pop up. The ones that I use are older Sevenstrand rings that I bought in bulk a long time ago for tuna fishing but Owner makes them too.

Here's a like to Melton's Tackle. They are a premium big game store so you can probably find them cheaper.

http://www.meltontackle.com/catalog/product.asp?product_id=13506