Therefore Abbey is probably the only place you are going to get one, but as its specially made it will be expensive
Website: http://www.ridingbitz.com/
the biggest i could find is 5.5" but its too small, the only six inch my local tack shop had was a mullen mouth rubber one, got it cheaply as its been at the back of the bit stand for ages but there is a high probability he will chomp his way through it,