Most any Tool Hire or such as Machine Mart, can and will sell you a thread gauge. BUT that ONLY determins what type of thread it IS. Best use a micrometer or other width/depth gauge to measure the diameter of the bolt. Better yet, happen you know the make of the rifle, call them up and ask them what type of bolt, its diameter & thread type is. That's by far and away the CHEAPEST way to do it.
Saddle Tramp.
PS, I've still got all my thread gauges, 10 in all, but there again, back when I began as an aprentice, there were a whole heap MORE thread types than are in use today. AND mine were ONLY used in heavy engineering and thers was and still is gauges for BA types???????????????????????????
PPS. You COULD take the stock bolt's you have with you, roughtly measure with a ruler, then look for same diameter and length, then try fitting present bolt's thread into replacement bolts threads, if the fit and the lengths right, your on a winner!