A complete library of medieval documents, called the Medieval Sourcebook may be found at the web. It focusses on the United States during the second half of the Nineteenth Century. A very useful and searchable index of books and source material at the web may be found at the On line Books Page . From here it is easy to check if the text you seek is available in electronic form. The computing centre of Oxford university offers an electronic textarchive at the web.
Texts are available in electronic form and at low costs.
The university of York maintains a site with source material regarding history of statistics , including several texts by Thomas Bayes, Laplace and Legendre. Stacy Langton maintains a page about his Euler project, containing several English transcriptions of source material regarding Euler . The Mathematics Genealogy Project contains a searchable database about the intellectual genealogy of mathematicians.