.html version copyright © 2010-2013 Henriette Kress.
You'll find .pdf versions here:
- Vol. 1-2 at Botanicus (scan from mobot) (beautiful scan).
The same, at the Biodiversity Heritage Library: Vol. 1 (1825) - Vol. 2 (1828).
- A google book scan on archive.org: Vol. 1-2 (readable text, botched pics).
- Google book scans: Vol. 1-2 - Vol. 1.
Vegetable Materia Medica of the U.S. or Medical Botany, 1818 (2nd ed. 1825), was written by William P. C. Barton, M. D.