I have been trying to get a pair of size-18 rock shoes. Does anyone build, or is anyone willing to build, a large shoe? I sport climb barefoot, and I am reaching the point (5.12) where it is extremely difficult to progress without better traction on my feet.
You are climbing 5.12 barefoot? Perhaps we should all take a lesson from you, and start relying more on talent and brass than rubber. It's worked before. In the Elbsandstein region of Saxony, Germany, barefoose climbers from Oliver Perry Smith a hundred years ago to Berndt Arnold in the 1970s have tackled steep sandstone walls up to 5.12. Are you a relation?
Per your problem, the market for kicks your size is diametrically opposed. Outside of clownshoes.net I couldn't find any shoes that would fit you. In fact, during my 34 years of climbing, the only person I've ever seen with dogs your size who climbed was a bodyguard for David Lee Roth, former swordsman and lead vocalist for Van Halen. (Rock on Eddie!) A custom cobbler made his monstro shoes, which I swear could have doubled as paddles, but that craftsman fell in love with demon rum and can't be bothered now.
Commercially, you can get some select shoes in size 15 from Acopa, Cava, Evolv, Five Ten and Mad Rock. Maybe an unlined leather shoe from one of these companies would stretch enough to be tolerable. Tony Puppo of the Rubber Room in Bishop recommends a pair of those fashionable rock-shoe look-alikes, such as the Puma Chalk or Scrill, and resoling them with sticky rubber, but even those in your size will be tough to find, even on Google.
That failing, according to the Society for Barefoot Living (for real, yo!), tossing out the shoes can improve your life. They cite an article in The Journal of the National Association of Chiropodists: Survey in China and India of Feet That Have Never Worn Shoes. The article states that 118 rickshaw coolies who pulled passengers at a run on cobblestone roads had healthier feet than people who went easier on their feet and wore shoes. Get naked! Of course, as usual, if you experience an erection lasting longer than four hours, seek medical attention.