Can't you measure up the one you have got to use as a sample? I have a few knocking about which may do but you need the thread diameter (I'm assuming it a metric size) and the overall length.
Failing that if you can find a bolt of the correct size, i.e. thread diameter and sufficient length of thread, you can cut to the required length and cut a suitable slot at one end for a flat bladed screwdriver. The front end of the grubscrew may have a pointed or hardened surface but for what you are using it for, one made from a bolt should be OK.