We like free...
LibreStock (www.librestock.com) is a good place to start and is a search engine that combines the content of 40 other free for private or business use photo library websites.
Usually the only limitation with images like these is that you can't re-distribute the images on another website or on items for sale. However its always a good idea to check the copyright notice to see if there are limitations or rules that must be observed when used e.g. some may require attribution on the website that the image is used on.
In general, this may be referred to as a 'creative commons' licence.
If you are gathering content for your website, you may also want to consider free stock videos.