Have you considered ordering from eBay. Should be available.

I don't think they are difficult from seed (but it is last in the growing season).

I can't believe I didn't think of that. I looked at seeds but didn't think anyone sold the rooted stalks!

They are bare root and I've never potted bare roots before. How do I plant them, just add garden soil or potting soil ?

For most plants, bare root is no big deal (there are a few where it would just be a bad idea). Just plant them in some sell-draining soil . You might add some perlite for better drainage.

I've grown lemongrass from seed and from grocery store stalks. Seed takes a while and easy to die off inside in my winter climate but stalks were stronger and are multiplying like crazy now.

Look for those plastic little clamshell packs since moisture/freshness of stalk is a concern for rooting. Try to find a piece that still has a bit of stalk below a joint. Peel away 1 leaf at a time from the joint til you see bumps, those are the root nodes and stick in water just to the bumps since you dont want top to rot. Change the water every other day. As the roots grow either the center will re-grow green shoots or new sides will come up. I tied a string around the top of mine and cup rim so I could suspend higher up with more water to allow the roots to grow longer and thicker before putting in dirt. Mist it a bit as it adjusts to growing again. After a few months as it gains outer stalk clumps and multiplies, the original will dry out.

plant nurseries sell them as starters