These might be hard to find at a hardware store, but we are happy to send some out to you. Any bike store will have the right size axle nut as well.
First, the axle nuts are all 15mm in size, meaning they can be tightened with a 15mm wrench or an adjustable wrench.
They are made to thread onto a 3/8-inch axle.
The threads per inch are as such:
For Around The Block Single Speed and 3-speed Bikes: 24 Threads per Inch
For Around The Block 7-Speed or 21-speed Bikes: 26 Threads per Inch