Why not head out to Rabao or to the jewelry shop in Jeuno, buy "cheap" mythril earrings (around 4500) and desynth into ingots (NQ, HQ1 = 1 ingot, HQ2 = 2 ingots). Takes lvl 47+ GS, but nice profit. So yeah, in a sense, this is "buying" from NPC...
The only way to make them from NPC bought materials is the ore recipe (goldsmith guild), and ore costs 1,500-3,000 gil there, although it rarely goes over 1,500 since everyone buys it at the AH. Profit varies by how much you pay for ingredients and how much you sell for, I'm leveling off the ore recipe now and buying for 1000-1200 at the AH for ore and selling ingots in singles in Jeuno for 5,000-6,000 each, making from a slim profit up to about 1,000 each.
Although I haven't tried it, cause I want to get a few more crafting levels under my belt before I start, Fatimah is giving me the first mythril ingot recipe (using mythril beastcoins) at goldsmithing skill 28.