Takuan is super easy to make at home.
Just take a daikon radish, peel it and cut into large sticks. Throw in a ziplock bak filled with kosher salt. Make sure the daikon is completely buried.
After 2-3 days (depending on how big your cuts are) take them out and rinse the salt off. Put in a ziplock bag with a mixture of mirin and rice vinegar- with a shiso leaf and an ume pickled plum if possible. If you don't have the shiso and ume it'll still be better than most store bought though. Let cure for a week.