Most olympians travel ALL. THE. TIME. Winters in Wellington, summers at Spruce meadows, HITS, Devon, Indoors, etc. Shows almost every week. Not to mention constant flights to Europe within those weeks. Its difficult to get a lesson in with any of them, although some may do clinics here and there. I would look into a hunter/jumper association for your state and see who the big show barns are. There might be a few grand prix riders who do lessons. Although there are some really spectacular trainers that you can learn quite a bit from even if a they don't do grand prix regularly.
Hi! I'm new here, but I have some contacts up in that area. For GP jumper riders in the greater Minneapolis area who teach lessons, I'd recommend Chris Underhill at Alpine Farms or Kim Barone at Raven Ridge Farm. :)