It's a weird bolt, but if you take one to a hardware store and use their thread & size gauge you'll be able to find one that will work.
Or go to the pick your part yard and pull one from a wreck. Will be kind of expensive for one - the local LKQ charges $0.50 for one, pluse $5 to get in. I have not had all that much success at Home Depot when looking for metric bolts. The OP could order one from Toyota, these are cheap even there, but they may not have it in stock: https://parts.longotoyota.com/oem-parts/toyota-ignition-coil-bolt-90105a0103
Our walking-distance Rona (lumber and hardware retailer chain store) has loose bolts and nuts, including a decent selection of metric. With that set up you can bring along one of the remaining bolts, test fit it on the metric nuts, then get the corresponding metric bolt. do keep in mind: the original-but-missing bolt IS close at hand; persevere at finding that “lost sheep”? I’m compelled to also note, just in case: Toyota specs the torque for those bolts in inch/pounds, not foot/pounds. Which are less by a factor of 12. A small but regular parade of priuschatters have attempted to torque those to that number of ft/lb’s, and snapped the heads off partway through the process. It’s simplest to appreciate they’re merely a light-duty hold-down bolt, snug them down gently.
If you dropped it, a magnet on a flex might find it. Or it might bounce somewhere and do some damage. Finding a replacement should be pretty easy. Take the other one out and go to the hardware store. They may have a thread checker or just use a nut to identify the thread pitch. Probably 1 or 1.5 It is probably a M6 bolt. I struggled with this stuff for years then bought a thread checker for metric and fractional on amazon for a couple bucks. Now my biggest problem is find where I put it!
Thanks for the input guys. t's an M6 25mm. They have them on Amazon and they delivered on the same day. (Amazon UK are wonderful) They were a fiver for ten.
Most of the failures to find a matching bolt at a hardware store, in my experience, are because either they just don't have that size, or the ones that they do have are only threaded on a region near the tip, where the application needs threads much farther up along the shaft of the bolt.
