Little Z - why don't you contact someone who sells the Mountain Horse Boots. Discuss the issue with them, and have them help you out. You can have them shipped to you, where you can try them on for size, return them if they don't work and try again.
There's no way I'd pay $1,000 for a pair of boots, I have a hard time even imagining forking out $500 for higher end boots you see in Dover Magazines or SmartPak........>.< ouch!
I hope you can work this out! You can try paddock boots and leather half chaps, discuss this with the college and see if these are permitted in the show ring.