You'd need a friend who doesn't ask questions. Collecting cum from a big horse can be kinda risky especially if the horse isn't really well behaved. It's being collected for a mare and often comes with a significant price tag. So, you'd need a friendly rancher with a trained horse and the right equipment, who knows you won't use it in a mare, and it's not the cum he was gonna sell.
in other words, while for some horses, getting them to cum for you can be very easy, for the most part, the situation around it doesn't lend itself to casual cum sales.