Sources for Bibtex entries

About 10 years ago I wrote a perl script called "bibweb" which uses MathSciNet to produce bibtex entries for references. If you have access to MathSciNet and you run

bibweb -c 'hartshorne;algebraic-geometry'

then one of its answers is

@book {MR0463157,
    AUTHOR = {Hartshorne, Robin},
     TITLE = {Algebraic geometry},
      NOTE = {Graduate Texts in Mathematics, No. 52},
 PUBLISHER = {Springer-Verlag},
   ADDRESS = {New York},
      YEAR = {1977},
     PAGES = {xvi+496},
      ISBN = {0-387-90244-9},
   MRCLASS = {14-01},
  MRNUMBER = {MR0463157 (57 \#3116)},
MRREVIEWER = {Robert Speiser},
}

It's not perfect, but it works for me. Download it at https://github.com/jhpalmieri/bibweb. It's free, so satisfaction guaranteed or double your money back.

I recommend using the AMS website MREF, located here.

EDIT : Another remark about your question. Don't worry too much about getting things like the printing date for a book correct (it changes every time they make a new printing run). Just make sure that you have the author, title, and edition in some standard format (and for papers, the journal, date, and page number). Every journal will reformat things into their house style and verify that your bibliographic entries are correct. Make those copy-editors work for their money!

Google Scholar will produce BibTeX entries if you turn on Show links to import to BibTeX in Scholar search preferences.