Measure it and ask your local fasteners supplier.
I live near a Cofast who have boxes and boxes of all sorts of permutations of metric bolts right there on the shelf including ones with an unthreaded ‘shoulder’ like shock bolts. Last time I went in amongst other bits and bobs I got a handful of stainless m5 with half unthreaded half threaded for a quid.
On that basis I can’t imagine a single long-ish stainless m6 or m8 bolt being any more than a quid, iirc cofast have a minimum ‘line price’ of a quid so you might even end up with three or four costing you the same as a single one.
