Water bottle bolts are typically 5 mm or 4 mm shaft. You can measure your existing ones with a caliper. A mm is .0394". They also have a # mm per thread, typically .8 mm on a 5 mm bolt. You can buy various lengths.
I usually buy these in the US from mcmaster.com
https://www.mcmaster.com/screws/rounded-head-screws/thread-size~m5/hex-drive-rounded-head-screws/
I buy stainless for a bike that sits in the rain a lot.
You can also buy them in phillips head or straight slot head if you prefer not to carry an allen wrench.
Freight is $9 Cleveland to Louisville so don't skimp on the # of products. 10 # or 1 # it is all the same. Don't buy from mcmaster overseas; UPS has a ~$25 customs loan fee.
You can buy on ebay with USPS shipping but cheap bolts often come from the reject bin with hydrogen embrittlement. They can fracture.
Amazon shipping is 50% higher than UPS but if you subscribed to their cable channel, is "free". I don't like soap operas that much.
BIke shops as modernbike.com or universalcycles.com have them but are way more expensive, usually selling 5 or 10 per package. Can save freight though if you need brake cable or chain parts in the same box.
You might be able to buy them out of the expensive bins @ rural king orscheln or TSC farm supply. They will be $1 each. The bulk bolt bins stop @ 6 mm at my local store.
