I haven't seen stakes that tall - and if there were, how in the devil would you transport them home?
I normally don't use a stake as high as my larger plants though - certainly not as high as my sunflowers. Not that mine ever got that big. Congrats on that BTW. How about a pic?
Mine ot about 10 -12 ft, and I tied them to the fence.
Just find the sturdiest, tallest stake you can - check the garden centers - and try that. It should do.