welcome to GoY. The genus has mostly herbaceous perennial plants that die down every autumn, but some are annual plants like Pixie and there are a few which are shrubby.I used to grow a shrubby one L. arboreus many years ago. The chamis de monte (Lupinus jaimehintoniana) of Oaxaca in Mexico, is as you know a true tree and grows up to 8 m (26 ft) tall. It has lilac flowers according to the books.

As for getting seed or small plants perhaps search on line for it. You will probably have trouble getting any from outside the USA through customs with out a phytosantiary certificate.

If you do find the seeds outside the US, use the small lots permit you can get online from www.aphis.usda to mail them. It's free. Plants, however, do require the phyto certificate and can cost a lot extra.