I know Keith Kiebler. He is in Illinois, southeast of Marion, IL. He is honest and reputable. My only caution would be, Keith rides fast. Obviously, for competitive riding. As such, you may not find his horses have the slow gait you may want for pleasure trail.
Finding a large, smooth gaited horse for trail is really tough. The TWH probably have more size to pick from, BUT, that's the last breed I'd look for in a trail horse. IMO, they have totally ruined this breed for pleasure trail, they are breeding for pace, in order to get the big lick horses. YES, there are a few pleasure breeders, but they are like finding hens teeth in a hay stack.
I'd look for the horses with a rack as their signature gait. Like the KMH, the Rockies, or the just simple no breed racking horses. My best gaited horses are of no particular breed, they just do it. I put absolutely no faith in a vet check, or any other professional checks, when it comes to having a smooth gait and naturally gaited. I prefer to have someone that rides gaited horses a lot(not necessarily a trainer) ride the horse for you, if it's too far to travel for a look see.
I'd also be very careful buying a horse that has been ridden a lot lately. Many gaited horses are smooth now, but after a lay off for the winter, or whatever, are not natually gaited enough to keep their gait.
If you can find a horse with some Cupid's Arrow blood(single footer/rack) some of them are large. I have a Cupid Arrow son that is 16.2 hds. That tends to be a trait of these horses. There was an EZD's Big Mac son for sale that was about 16 hds. EZD's Big Mac is a son of Cupid's Arrow. Most all of these horses are very naturally gaited.
Here's a picture of a 16.2 hd, Cupid's Arrrow son, out of a McCurdy mare.