The Keller correctors are available exclusively through Astro Systeme Austria. I have the 3" version in my scope but a 4" version will be released soon with a 60mm+ corrected FOV for the large format chips that are becoming more popular now.
I purchased my ASA N12 OTA (with 3" Keller corrector) from Astro Systeme Austria.
http://www.astrosysteme.at/ikreator/asa/cms_pub/lang_en
Mike