You can just use Google.
But most of what you find will be covered by copyright (contrary to popular belief, just because it's on the Internet does NOT mean it's in the Public Domain). Here are some sources that you can use to locate images that you CAN use. This may be because they are old enough to be in the Public Domain, because the owner has granted public use of them, or they are part of a database that the Libraries' have subscribed to specifically for this purpose.
An example of how one can use an image that you find in one of the sources listed on the left is the image in the banner at the top of this page. I found that image in the Wikimedia Commons. The original is here
where you will see that the original photographer - Joe Mabel - has said
I have 'remixed' and 'shared' the image, and I am giving him attribution here and others are free to take my 'remix' and use it if they have a way in which it will fit their needs.
UWM's Digital Collections have a statement about use of the images contained. It reads
Reproduction and Use
Please use this form (not duplicated here) to submit an order for high-resolution digital reproductions of images included in the UWM Libraries Digital Collections. A reproduction fee is charged for high-resolution copies in TIFF.
The low-resolution images available from the UWM Libraries Digital Collections website may be copied by individuals or libraries for personal use, research, teaching or any “fair use” as defined by copyright law. Low resolution images can be downloaded for no charge by clicking on the “download” button at the upper right hand corner of the image and saving to your local computer.
Use fees are charged in addition to reproduction fees when images are published or redistributed. Use fees are negotiated on an individual basis. Requests to publish, exhibit, or redistribute the images included in the UWM Libraries Digital Collections must be obtained in writing. Please contact the UWM Libraries staff at email@example.com for a permission form.