Can't say I have ever seen bolts with heads like that. One option, if you have a Tig would be threaded rod and weld a dome nut on. Another option could be to get them custom made, Hague Fasteners specialise in such things but not a clue how much it would be.
Loads of applications where they are used on rocker covers but the pickings are slim. As above I think ht studs or threaded rod will probably be more cost effective .
If it’s bolts you need and the tensile strength is an issue, 8.8 threaded rod should be obtainable from a decent nut and bolt supplier. As should dome nuts. Stud lock or bearing fit should marry them together. If unthreaded shanks are needed for a shear application, engineers studs might suit instead of the threaded rod. Again available from a decent nut and bolt stockist.
If access to a decent nut and bolt stockist is the problem, Screwfix (and toolstation) list stainless metric dome nuts.