With regard to Bont's: They used to make 51 and 52 as stock sizes. So, they probably have stock lasts, but, just don't build them as part of the product line. You might not have to go through the cost of sock molding, etc. It "might" simply be a matter of them building to order your size on one of their pre-existing lasts. It's worth contacting them directly.
Lake has also made extremely large sizes from time to time and may have lasts on the shelf. Again, worth contacting them directly.
I have both Sidi's and Shimano. The Sidi's are certainly nicer and stiffer. But, considerably more expensive. For my current use them, abuse them and wear them out training shoes I couldn't justify spending the additional money and ended up with the Shimano.
If you contact Bont and Lake, please, come back and let us know what you learn. The options for 51/52 are few.
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