How to size fork boots?

I'm looking to get fork boots for my kz400.
Fork tube 33mm. fork lower 56mm. Length needed when fork fully extended 240mm.
Since it is hard to find exact size, do you buy a smaller size and stretch them on?
How much will they stretch?

You mean these? Too small they will crack.
Steve
30mm Rubber Fork Boots Gators @ 33mm @ Kawasaki KX305 KZ400 KZ440 LTD BN125 Z400
 

Thanks SWest. The size was wrong, but you got me to the right vender.
This is what I need.