I'm going to be honest about something--in a serious novel, I wouldn't try to spell out the way a horse's snort sounds. It's different if you are trying for comic relief (which you may be) but I tend to think horse noises are a distraction. I really haven't a clue where to find the information. I found a site that has wav files of horse sounds but that wouldn't work very well unless you have an interactive novel. Sorry.