ZoteroBib (brought to you by the same team that created Zotero) is "a free service that helps you build a bibliography instantly from any computer or device, without creating an account or installing any software. If you just need to create a one-off bibliography for a paper, ZoteroBib is the quickest and easiest way to do it." Just type your source URLs, DOIs, ISBNs, PMIDs, or even source titles in the box. ZoteroBib will find the bibliographic information for you and add it to an online bibliography (in any of a range of citation styles). When ready, copy the bibliography to your clipboard and paste it into your paper. For more information, see their FAQ page.
Type in your DOI, ISBN, etc. in the search box. ZoteroBib will (in most cases) find your citation for you.
When ready, just copy your bibliography to the clipboard. Then paste it into your paper.
Copy the URL to your bibliography to come back and add sources later.