In 2017 I grew flax seed. The easiest seed to find was in the bulk section of a local grocery store. I bought a few pounds of both organic brown and yellow varieties. I now know that food varieties will not make the longest fibers nor are they even the right specie. I also planted the seeds in a less than ideal configuration. Sometimes learning is done the hard way. I don't have any printed sources that talk about growing or processing linen and I was reticent to try to find any but those I could find online for free.
A better seed source would be Linum usitatissimum from the linked garden center or another reputable seller and planting should be done in a large garden (rather than strips) so it stands up against itself which may discourage branching.
Read more below for this crazy trial.