Go to any tack store/catalog that sells supplies for harness horses. These fit quite snug, don't flop around and cover the heels/bulbs of the front feet for horses that over reach. Your horse might hit them, but it can't step on it and flip/trip.
My horse and I had a wreck last night while schooling dressage thanks to our bell boots. My poor horse stepped on a front bell boot with her hind and went head over heels. Right over, her hip just glanced off of mine (so so lucky) and my poor saddle is ok just filthy and my bridle too as she did a face plant. We both walked away covered in dirt but I had no idea that could happen. Granted, she was being at bit naughty and the step she took that did it was leading into a spook/scoot. She a definite over-reacher but I’d rather deal with cut up heal bulbs or a pulled shoe than ever risk that again. Thank god we didn’t get seriously hurt. She was in the Italian rubber pull on bell boots, I’ve had them completely rip or pull off before but not this time, maybe because they were newer and not weathered and worn. Dressage is so dangerous! Imagine if I was jumping...don't want to.
I had a similar problem with bell boots while schooling a stadium course with my guy. We were jumping left to right on a diagonal...had landed...he did a flying lead change, and BOOM, he was down on his side. My trainer saw the whole thing happen, and she said he caught his back hoof on the bell boots. I have since started using a size small bell boot instead. They look kind of ridiculous when he's standing (he's a 15.2 OTTB), but they no longer graze the ground, and they make me feel safer.