Other option is a JTek shiftmate
http://www.cyclingabout.com/jtek-shiftmate-mix-mtb-cassettes-with-sti-road-shifters-to-lower-your-gearing/
Or a Lindarets Goat Link.
Or buy some second hand 9/10/11 speed Shimano MTB rear mechs and see if you can get one of them to work acceptably.
@macaroon started
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Other option is a JTek shiftmate
http://www.cyclingabout.com/jtek-shiftmate-mix-mtb-cassettes-with-sti-road-shifters-to-lower-your-gearing/
Or a Lindarets Goat Link.
Or buy some second hand 9/10/11 speed Shimano MTB rear mechs and see if you can get one of them to work acceptably.