I think you've been unlucky. I've always just bought whatever I could find at what I thought was a reasonable price rather than sticking to brand names and I've never had a problem.
I'd just head on over to evil bay and buy whatever... but if you want to spend more I've always found Speedgraphic to be a good source of bits and pieces.
http://www.speedgraphic.co.uk/search.html?searchterm=stepping ring
I second that! Have a look here: http://srb-photographic.co.uk/SRB bits and pieces are usually good quality and reasonably priced.