It is probably due to lack of demand. Also, as I understand it, borage is a medicinal herb not culinary.
Google online seed venders. you will find it. I think I have seen PIKE nurseries sell the seeds and the plant.

Thank you. So I guess if I want to plant some I guess I'll have to order them on line.I might could find a plant in a local nursery,never checked to see.

Or you can try asking for some on one of the seed exchange groups here.
It will reseed itself once you get it going.

Borage is as easy to grow from seed as marigolds and calendula. Buy or trade for seeds. Any of the herb seed vendors will carry it. Try one of these:

Richters
Horizon Herbs
Nichols Garden Nursery
Pinetree Seeds
Johnny Seeds

FataMorgana

Is it true that it can be invasive? Will it tolerate some shade?

Self-seeds like crazy but is very easy to pull up and get rid of whatever seedlings you don't want. It prefers full sun and may tolerate some light shade but it will probably stretch and sprawl for the sun if put someplace with less than full sun.

FataMorgana

IT IS CONFIRMED.
As I mentioned before, PIKE nurseris might have the seeds.
Today I was in a PIKE store and I bought Borage seeds and aleady planted some.

As fatamorgana says, just make sure you pull it before it self-seeds. The flowers are very pretty and have a mild cucumber flavor, nice in punch bowls. The leaves taste like cucumber too but are a little fuzzy. If you want cucumber flavored leaves for salads, grow salad burnet, a (sometimes short-lived) perennial.

Borage does have beautiful flowers but they are downward facing so they are not always the most obvious. And as nygardener says, they are pretty in punchbowls and as salad toppers. I read once that the Old Masters would match the color of the Madonna's robes in their paintings to borage flowers. I'm not sure how true that is but it does tell you the color of the flowers.

Borage's flowers are great bee magnets and I always leave some of the self-seeded plants in and around the vegetable garden to attract pollinators.

FataMorgana

A nice idea is to freeze individual flowers into ice cubes and use them in summer punches, especially, of course, in Pimms.

I was just going to comment, flora, that it's so nice in iced tea, frozen or not. Also on lemony sherbet or ice cream.

I cannot wait for my borage seeds to germinate and grow.
Originally, I was interested in its medicinal benefits, which is said to be: MOOD ENHANCER/LIFTER, SKIN CONDITIONER, and LOWER HIGH BODY TEMPERATURE.
Thanks to OP, because this thread I made the move to grow borage.

My borage tastes more like fish oil than cucumber. But it does self seed readily, and the bees love it.