Not sure of your location, but Home Depot or Lowes should have a common bolt like this. If not look for a Bolt and Fastener specialty store in your town. This is where I find most of my hard to find metric stuff. Then there is McMaster-Carr.
I remember the bolt that you are talking about with the oddball 11mm head. When installing my AC unit, I had to remove a seat for the first time and found that I had to grind down the wall thickness of a perfectly good 11 mm socket in order for it to reach down inside of the GT seat rail and fit over the bolt head. Good Luck!
Refer to mcmaster(dot)com.
Head height for a M8 hex head bolt is is 5.3 mm.
Stardand allen head socket head cap screw should / may fit in the Opel GT seat tracks, as noted.
If not, a button head or low profile socket head cap screw should fit.
See attached McMaster drawings for comparison dimensions.
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