Have you checked with the standard relief valve vendors. They will usually provide you with any flanges you want, whether you want them depends on the price and delivery they quote versus what you want to pay and when you need it ;-).
If you don't mind a question, a 10,000 lb RTJ valve on liquid N2 seems somewhat unusual. Why do you need this particular flange? Can you use a transistion piece with a 10,000 API flange welded to a ANSI flange?
