[3xNinjed, but posting anyway...]
Just started a new game due to a huge overflow of seeds because I didn't realise they needed to be bagged prior to barreling.
I've made sure this time to plant pig tails asap with the intention of getting a base textiles industry up. I planted 5 and appear to have harvested 9 pig tails and now i'm out of seeds. My plump helmets seem to have no end of seeds but have no idea how to get more pig tail seeds.
Seeds can be obtained (where possible, which includes pig tail) as the product of plants being eaten/brewed/milled/farm-processed, according to whatever their plant can be used for.
Not cooking, although that shouldn't be the issue given you can't cook Pig Tails.
If you're wanting your cloth (and my memory tells me that there's nothing else that can be done with Pig Tails.. apart from brewing, which will also give you seeds), process the plants in the Farmer's Workshop. You should get at least one seed per plant, which will get grown in the fields (Summer/Autmun only?[1]) to give you more to process, and should build up your stocks.
Just make sure that you don't have the seeds available to cook (by default it shouldn't be, but that would use up seeds to no gain except emergency food), and I think that un-harvested plants can rot in the fields and similarly lose the ability for seeds, so turning off "All Dwarves Harvest" might be a bad idea unless you have enough farmers, and even with that option on you
may find everyone too busy to help out if you're like me and trying to keep everyone doing something else.
[1] Strange how above-ground plants can be grown[2] all year round, yet subterranean ones are sensitive to the seasons...
[2] Assuming they can at all. My glacier embarks refuse to give me above-ground plants, even when I scrape back to soil.