Usually the nearest town. I’ve found that the rule I was taught held true, subtract 4 degrees for every 1,000 feet of elevation gain. So if the town says it will be 30 at night, and they are at 7k ft, and you camp at 10k ft, expect it to get down to about 18 at night. And keep in mind local geography can affect that. If you are in a draw at 10k, and the mountain top above you has snow on it, that cold air may flow downhill and be even colder than you expected.
Weather Underground has historic weather data for specific towns, look up the town elevation and then do the math above for your likely camping elevations.
Hope that gets you in the ballpark!