You can get BibTeX entries in a few different ways:
A) From any database
B) Hand coding entries into your BibTeX database. Not recommended but sometimes unavoidable, especially for unindexed literature or other uncommon resource types.
C) Use a tool:
Most of the commonly used reference/citation managers (EndNote, Zotero, Mendeley, etc.) will have an option to export to a BibTeX database. It is highly recommended that you use one. Citation libraries can also be exported from one manager to another if you want to try different software.
There are also citation managers designed to work specially with BibTeX. Here are a couple: