I've never found them in swamps near the starting stones, and I've always found them in swamps far away. Since crypts are also uncommon near the start location, my basic strategy is to just go by boat as far as I can and scout swamps from the coast for crypts.

Bonemass has always been a long trip away too, so it has happened naturally that I tend to find turnip seeds in the same swamp as Bonemass, if I haven't found the seeds earlier.

Never seen an actual turnip in the wild. I thought you had to pick flowers and plant the seeds first.