I just put one in my MS6 3 weeks ago, during the last oil change. It's the same size (dia and thread) as for other Mazdas. I got it off eBay for about $6 shipped from Hong Kong (bought 2, one for my brother who has a Mazda5) with combined shipping.
The bolt head is different (IIRC it was 14mm, noticeably smaller than the stock one on the MS6, which I think was 17mm or 19mm?) but otherwise it went on fine. One item to note though, the one I got was made from aluminum, with a cylindriacl steel magnet inserted into the end (apparently pressed in as there wasn't any lock screw). If you get the same type I would strongly recommend doing oil changes yourself, to avoid ham fisted oil change "technicians" from stripping the bolt head.
I'm unsure if I will keep the bolt on the car long term. At this time I'm going to see if it picks up anything when I change the oil next. Depending on the result I may just reinstall the stock one.
BTW, if any of you are also active on the Mazda6.club forum I'm SuperCar Tastes - Skoka Budget over there.