I can only speak of Shimano. To my understanding, and this most certainly might have changed since, you can get a 12-30 in both the Tiagra and Ultegra lineup. The Tiagra came first and I purchased one last year off eBay as a trial to see if it would work with my Dura-Ace and Ultegra mid-length cage rear derailleurs. The Ultegra 12-30 was "announced" and appeared on the Shimano web pages, but nobody had any yet. I tried the Tiagra 12-30 out on my bike, the shifting and chain length were fine, (I had previously used a 11-28). So, I sold the Tiagra to a fellow BF member and went on the hunt for an Ultegra 12-30. Of course, nobody had any. Not even eBay vendors.

Armed with a printout of the Shimano spec sheet, I went into one of my favorite LBS, seeking to order one. It is just a one-person shop. The fellow is the owner, does sales, and all the repairs/service. He does it all. He did not believe that a 12-30 was available, and it so happened that one of his suppliers was in the shop at the same time. The rep said he would check his inventory, and after booting his laptop, said that he had ten of them. The shop ordered two, one for me and another for his shelf. When they came in, I went ahead and got both of them. (I think he charged me $100 each, tax included.)

This was about a year ago, or just under. Today, most shops still do not stock a 12-30. I did get one about a month ago off eBay. It was a decent price also, $75 with free shipping. If you go with Tiagra, they are less than half that price, but personally, I didn't like the Tiagra. There is a world of difference between the two lines.

So, they are available. You just have to hunt around. Readily available on eBay, special order from your LBS. If you go to your LBS, be sure to take a printout of the Shimano spec sheet so that they cannot refute they even exist. Good luck. They are a nice cassette, (I'm not really too concerned about jumps between cogs).

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edited to add:

http://www.shimano.com/publish/conte...0.-type-..html

PDF spec sheets at top/right of this page under "Technical Documents".