If you want to stay with a Seiko, the 6138 movement is the most readily available chrono movement. This eBay search shows quite a few designs: http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l2736&_nkw=Seiko+6138
Outside of Seiko, my favourite is this: http://www.steinhartwatches.de/en/Nav-B-Chrono-47mm-black,397.html bit spendy though
@Soul started
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If you want to stay with a Seiko, the 6138 movement is the most readily available chrono movement.
This eBay search shows quite a few designs: http://www.ebay.co.uk/sch/i.html?_from=R40&_trksid=m570.l2736&_nkw=Seiko+6138
Outside of Seiko, my favourite is this: http://www.steinhartwatches.de/en/Nav-B-Chrono-47mm-black,397.html bit spendy though