You can find Erasmus Darwin's work at the Internet Archive Open-Access Text Archive - a wonderful resource with over 200,000 items, most of them out of copyright or otherwise in the public domain.
Here are links to the Temple of Nature, and Zoonomia. Select the format you want to download from the panel on the left.
You can also find Erasmus at Project Gutenberg, the first producer of free electronic books. They have Zoonomia and the Botanic Garden. They also have most of Charles Darwin's books, including the more obscure ones.
Whilst I'm pointing out resources I should also mention Librivox, which provides free audiobooks of books that are in the public domain. This includes a nice reading of Darwin's Origin of Species. (Full title: On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life.)