With extremely few exceptions, patent illustrations are copyright, permission, and royalty free. Manipulate them into whatever you want. You will spend more time finding the right image with patents than with a pay-service, however, there is that word free to keep you motivated.
Start with Google's patent searching engine. They provide a thumbnail of the first or most prominent illustration of the patent in their search results. This is key in looking for design: search with the words ornamental or design. Motorcycle AND ornamental. Cat AND design. What you'll pull up most are design patents which is what you want.
If you find a patent with great illustrations you can download its file and start Photoshopping. I prefer downloading directly from the United States Patent and Trademark Office. In my experience, these are 300 d.p.i. files which work out great for producing line art.
The illustration below is from either a Kawasaki or BMV design patent. It was clutter free and needed only some work on the front tire to make it into the logo I was thinking of.
I brought the image into Photoshop Elements and created a second layer for the text. As an aside, I find today's Elements works very well for us non-professionals. It now has features like typing on a line that were unavailable in a low cost program years ago. Getting back to the illustration, my intention was to create an image that would work as a business card logo. Scrunching an image this size to business card dimensions could only work out well.
As you can see, before resizing,I added red to make the logo pop. I'm not always committed to black and white. In my next posts I will expand on patent searching to include Canadian patents and the ones belonging to the European Union. Again, look for design patents and happy hunting.
Update! Want an easy intro to patent illustrations? Just go to google and type in the search terms "united states patent illustrations drawings". Although most of the images returned are not linked to the USPTO, you may find enough information to track down the drawing you are interested in.