Where can I download a large sample bibliography collection in BibTeX?
For your purposes, I would highly recommend you to use The Collection of
Computer Science Bibliographies by Alf-Christian Achilles. This extensive collection contains 3M+ references on the various CS subjects (grouped in about 1500 collections) and, besides offering search and browse interfaces, allows one to download the actual bibliographic data in BibTeX format - just select a particular bibliography and you will see the links to the source files - uncompressed and/or zipped.

P.S. Don't forget to acknowledge the value of this resource to the maintainer of this meta-collection (a thank you note will do) and, perhaps, even attribute the source, if your software will be citable.