Don't know about where to buy, but I would recommend planting seed. Is very easy and yes, it does self sow quite readily.

Direct seeding works the best.Borage doesn't transplant very well so they say thats why I have never found it anywhere in Hampshire/Franklin County.
Though I started some seeds in my garden years ago and I do transplant the self sowen seeds with no problems about 90%of the time wqhen moving from one part of the garden to another.Just dont remove the surrounding soil from the roots.
You may also start them in peatpots inside and plant pot and all when the soil is warm enough.

Or if you want wait and we can do a plant swap.