Provisioning crafting writs only.

The 7 fragments of the recipe are only dropped as rare rewards from provisioner writs. Any tier writ could drop one, but devs have said they are more likely at higher tiers.

From my experience, I would guesstimate the tier 6 drop rate is maybe 2 percentish. Not great, but the good news is that people are also selling the fragments they don´t need at the guild vendors. I've seen them as low as 30,000 and they may go lower in time. Which means you can "farm" fragments by doing anything that gets you lots of gold.

Your best bet, then, is to max out the odds from writs by having all your alts do the provisioner writ daily, and stockpile the gold you get from those writs and from adventuring. By the time you've gotten three or four fragments from writs, you're likely to have enough money to just buy the rest of them, and you're done.

Do the fragments just turn into a complete one automatically?

When you have all seven fragments, you "use" one and all seven are automatically converted to the complete recipe. You can then "use" the complete version to learn the recipe, or sell it.

On PC I've seen the complete recipe available for 240,000g in guild stores, with individual fragments as low as 30k (confirming what @robeauch says).

I got one today out of sheer luck and I'm not even that high yet. Went to sell it, but prices are down near 12k, would rather keep a recipie I'll never see again than get a measly 12k quick.