My tall boots from Sergio Grasso are the most comfortable I've ever walked/ridden/ran after loose horses/trekked through the mud/cleaned stalls/gone to lunch in! They come in every variety of size imaginable AND are FEI approved. I wear mine every single day. You will not be sorry that you bought these boots. 
 
These lovely boots feature the following: 
Nappa calfskin upper and lining
SG Ultratech calf panel ensures perfect grip in the saddle
Rear reinforced elastic
Rear zip
Zip protection and spur rest
Spanish cut
ProComf.Evo® outsole technology